replace `Speech2TextTokenizer` by `Speech2TextFeatureExtractor` in some docstrings
# What does this PR do?

It seems to me that fbank features can be returned by an `FeatureExtractor` and not by a `Tokenizer`. This PR proposes to make some changes in the docstrings that mention that `Speech2TextTokenizer` should be used to return fbank features. 

It seems to me that among the 3 models - `Speech2Text`, `Speech2Text2` and `SpeechEncoderDecoder` -  only `Speech2Text` implements a feature extractor and I therefore deduce that the same feature extractor class that is used for the 3 models.

## Before submitting
- [x] 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](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? 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](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] 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. As it concerns audio related model, I would love to have your feedback @patrickvonplaten , @anton-l  or @patil-suraj 
_The documentation is not available anymore as the PR was closed or merged._