[Doc] Fix mistakes in docs for Trainer checkpointing backends

[kurtamohler]

Description

Fix documentation for Trainer checkpointing, which mistakenly said that torchsnapshot is the default backend, when torch is actually the default, as seen here.

Also fix incorrect syntax for the hyperlink to the torchsnapshot repo.

Motivation and Context

close #2075

• [x] Someone has raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds core functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (update in the documentation)
• [ ] Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [x] I have read the CONTRIBUTION guide (required)
• [x] My change requires a change to the documentation.
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [x] I have updated the documentation accordingly.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2285

• :page_facing_up: Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

:x: 1 New Failure, 8 Pending, 16 Unrelated Failures

As of commit 6cf8ee0c9c5434fc35806ba922ab949a10ed1384 with merge base 35663887d92419ec7eb326ec5ee4a1894b37d1d8 ():

NEW FAILURE - The following job has failed:

* [Continuous Benchmark (PR) / CPU Pytest benchmark](https://hud.pytorch.org/pr/pytorch/rl/2285#27255147589) ([gh](https://github.com/pytorch/rl/actions/runs/9865036725/job/27255147589)) `Workflow failed! Resource not accessible by integration`

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

* [Examples Tests on Linux / tests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255151874) ([gh](https://github.com/pytorch/rl/actions/runs/9865036758/job/27255151874)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Generate documentation / build-docs (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255149131) ([gh](https://github.com/pytorch/rl/actions/runs/9865036748/job/27255149131)) ([similar failure](https://hud.pytorch.org/pytorch/rl/commit/6cf8ee0c9c5434fc35806ba922ab949a10ed1384#27255161964)) * [Habitat Tests on Linux / tests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255149146) ([gh](https://github.com/pytorch/rl/actions/runs/9865036737/job/27255149146)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Libs Tests on Linux / unittests-gym (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255161964) ([gh](https://github.com/pytorch/rl/actions/runs/9865036772/job/27255161964)) (detected as infra flaky with no log or failing log classifier) * [Libs Tests on Linux / unittests-sklearn (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255162660) ([gh](https://github.com/pytorch/rl/actions/runs/9865036772/job/27255162660)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Lint / c-source / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255148381) ([gh](https://github.com/pytorch/rl/actions/runs/9865036741/job/27255148381)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Lint / python-source-and-configs / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255148838) ([gh](https://github.com/pytorch/rl/actions/runs/9865036741/job/27255148838)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [RLHF Tests on Linux / unittests (3.9, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255149662) ([gh](https://github.com/pytorch/rl/actions/runs/9865036743/job/27255149662)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-cpu (3.10) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255150200) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255150200)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-cpu (3.11) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255150625) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255150625)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-cpu (3.8) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255151308) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255151308)) (detected as infra flaky with no log or failing log classifier) * [Unit-tests on Linux / tests-cpu (3.9) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255152187) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255152187)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-gpu (3.10, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255153314) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255153314)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-olddeps (3.8, 11.6) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255152690) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255152690)) (matched **linux** rule in [flaky-rules.json](https://github.com/pytorch/test-infra/blob/generated-stats/stats/flaky-rules.json)) `The process '/usr/bin/git' failed with exit code 128` * [Unit-tests on Linux / tests-optdeps (3.10, 12.1) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255153729) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255153729)) (detected as infra flaky with no log or failing log classifier) * [Unit-tests on Linux / tests-stable-gpu (3.10, 11.8) / linux-job](https://hud.pytorch.org/pr/pytorch/rl/2285#27255154443) ([gh](https://github.com/pytorch/rl/actions/runs/9865036755/job/27255154443)) (detected as infra flaky with no log or failing log classifier)

This comment was automatically generated by Dr. CI and updates every 15 minutes.