Fixed docstring formatting
# What does this PR do?
Fixes the formatting in docstring for whisper model.
Fixes #22052 

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] 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. Issue #22052 
- [x] 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).
- [x] Did you write any new necessary tests?

## Who can review?
 @sgugger, @stevhliu and @MKhalusova

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22067). All of your documentation changes will be reflected on that endpoint.