Fix some doctests after PR 15775
# What does this PR do?

After PR #15775, we need to either update some expected values or specify `skip_special_tokens=True`.

I am not very comfortable to use `skip_special_tokens=True` for `PT_QUESTION_ANSWERING_SAMPLE` in `doc.py`, as it might fail other tests. We will have to run the doctest manually to see if everything is fine.

(A lazy way is not to use this argument, but just to update the expected values)

#### update
I launched doctest [here](https://github.com/huggingface/transformers/actions/runs/3384706275). The tests with `ForQuestionAnswering` all pass, so we are good!

_The documentation is not available anymore as the PR was closed or merged._<|||||>Sorry, I always missed the last piece. Added the final commit to fix `docs/source/en/model_doc/speech_to_text.mdx`.


https://github.com/huggingface/transformers/pull/20036/commits/e6c4bc5f45fee4a8d3997bc4c04896fac1c25284