Attention mask fixed. Documentation: @sgugger
Author: kant
Pull request: yes

# What does this PR do?
* Attention mask fixed (line 217)
* typo fixed (paragraph 326)

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Fixes # (issue)
No previous issue related

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?
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@Narsil 
@ydshieh 
@sgugger 
@omarespejel 
@patrickvonplaten 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19443). All of your documentation changes will be reflected on that endpoint.

Same as your other PRs, there is an issue with your CircleCI permissions, the tests won't run.
Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-)?

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.