Fix OneFormer Docstrings
# What does this PR do?

Fixes docstrings for OneFormer.

- [x] Checked that all the doctests passed for the following command:

```bash
python3 -m pytest -v --make-reports doc_tests_gpu --doctest-modules src/transformers/models/oneformer/ -sv --doctest-continue-on-failure --doctest-glob="*.mdx"
```
## 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?

@ydshieh 
_The documentation is not available anymore as the PR was closed or merged._