Fix typo in error message in generation_utils
Author: regisss
Pull request: yes

# What does this PR do?

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This PR fixes a typo in an error message in `src/transformers/generation_utils.py`.


## 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?


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Comments:
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