Fixed the docstring and type hint for forced_decoder_ids option in Ge…
# What does this PR do?

This PR fixes #19602 where the docstring and type hint for forced_decoder_ids option in GenerationMixin.generate were inconsistent with the actual implementation.

Fixes #19602

## 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?

@gante has suggested me to send a PR for the issue #19602. 

_The documentation is not available anymore as the PR was closed or merged._<|||||>@gante Thanks for the comment! I've taken in your suggestion in the new commit. Please proceed with the merge if it is looking good.<|||||>@koreyou Awesome, thank you for the changes! I will merge as soon as CI turns to green :)