Replace BloomTokenizer by BloomTokenizerFast in doc
Author: regisss
Pull request: yes

# What does this PR do?

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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:
```shell
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
- [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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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

CI passed, pinging @sgugger for final approval