Missing `return_dict` in Doc example
# What does this PR do?
Fixes a crash in [Summary of the tasks](https://huggingface.co/transformers/task_summary.html) documentation, by adding `return_dict=True` to the model() function, as we need `start_logits` and `end_logits` afterwards.

## Fixes
[Issue 9043](https://github.com/huggingface/transformers/issues/9043) (reproduced in 4.2.1)

## Who can review?
@LysandreJik @sgugger

Hello! Are you sure you checked in v4.2.1? I just checked on both `master` and v4.2.1 and the code executes (as it should!).

The `return_dict` was set to be `True` by default in v4.0.0.<|||||>My bad, I was in the wrong pyenv. Closing the PR.