README: minor improvements (backport #25)

[mergify[bot]]

🦟 Bug fix

Summary

This makes small improvements to the README:

• https://github.com/gazebosim/gz-usd/commit/8ec2acfcea84d0f51875962a2a167d61842e6d8d: Refer to the current GitHub repository name (PixarAnimationStudios/OpenUSD instead of PixarAnimationStudios/USD)
• https://github.com/gazebosim/gz-usd/commit/60125d35846844859b1dba00e02a8609c48ae5df: recommend defining a USD_PATH environment variable to make it easier to copy/paste some configuration commands

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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This is an automatic backport of pull request #25 done by Mergify.

[mergify[bot]]

Cherry-pick of 6548a11e321eb613fd3c867ec82a4a7ef0b51349 has failed:

On branch mergify/bp/garden/pr-25
Your branch is up to date with 'origin/garden'.

You are currently cherry-picking commit 6548a11.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   README.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally