Fix typo in BERT tokenization file
Author: qqaatw
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## 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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Comments:
Thank you for your PR! The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15228). All of your documentation changes will be reflected on that endpoint.

I was actually going to work on that :-)
Thanks for fixing! You can see the doc after your fix [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15228/en/model_doc/bert#transformers.BertTokenizerFast), whereas before had a problem as seen [here](https://huggingface.co/docs/transformers/master/en/model_doc/bert#transformers.BertTokenizerFast) (we just need to fix the link displayed in the comment above).

New bot looks great :)

Great job merging this PR! the documentation will now be removed from the staging environment.