Fix: typo in FINE_TUNE_XLSR_WAV2VEC2.md
Author: qqpann
Pull request: yes

Fix typo.

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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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- [ ] Did you write any new necessary tests?


## Who can review?

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This is a simple typo fix. Could you review it @sgugger ?

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