[Speech2Text Doc] Fix docs

[patrickvonplaten]

What does this PR do?

Fixes Speech to Text model docs and add tests

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
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• Did you write any new necessary tests?

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[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[ydshieh]

Thanks! Left a few comments.

I didn't run it locally, since it looks like there is currently some dataset issue?

[patrickvonplaten]

Great review - thanks @ydshieh

[sgugger]

Thanks for fixing!