Fixed typos in comments of OPTDecoderLayer
# What does this PR do?
Fixed typos in comments of OPTDecoderLayer ( used in Meta OPT models )

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

@sgugger 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18909). All of your documentation changes will be reflected on that endpoint.<|||||>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.