Fix a typo in preprocessing docs
Author: h4iku
Pull request: yes

# What does this PR do?

Fixes a simple typo in preprocessing documentation: mode -> model.

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## 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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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Comments:
Thanks a lot!