What does this PR do?

Fixes docstrings for OneFormer.

• Checked that all the doctests passed for the following command:

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

• 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,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? 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, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ydshieh