Fixes typo in `modeling_speech_to_text`
Author: mishig25
Pull request: yes

# What does this PR do?

@sgugger @patrickvonplaten please let me know if the changed line is a bug or a valid documentation 👍 

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Fixes # (issue)


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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