Replace BloomTokenizer by BloomTokenizerFast in doc

[regisss]

What does this PR do?

The current documentation examples for BLOOM propose to import its tokenizer as follows: from transformers import BloomTokenizer. This returns the following error with version 4.20.1:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: cannot import name 'BloomTokenizer' from 'transformers' (/usr/local/lib/python3.8/dist-packages/transformers/__init__.py)

because BloomTokenizer is actually not defined anywhere. This PR replaces it in the doc by BloomTokenizerFast.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[younesbelkada]

LGTM thanks for spotting this ! 🚀

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[regisss]

CI passed, pinging @sgugger for final approval

[sgugger]

Thanks for fixing!