Fix backend doc

[pytorch-bot[bot]]

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/3686

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:white_check_mark: You can merge normally! (2 Unrelated Failures)

As of commit 2920cf89058e9ca9ab1bd401eeee3f0c949b9022 with merge base 6e265157466bbdae7625095516f8a5c9de706427 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

* [Unittests on Windows CPU / unittests-windows-cpu / windows-job](https://hud.pytorch.org/pr/pytorch/audio/3686#18194678251) ([gh](https://github.com/pytorch/audio/actions/runs/6696606699/job/18194678251))

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

* [Unittests on Windows GPU / unittests-windows-gpu / windows-job](https://hud.pytorch.org/pr/pytorch/audio/3686#18194678752) ([gh](https://github.com/pytorch/audio/actions/runs/6696606738/job/18194678752))

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