fix parameter name in docstring
# What does this PR do?
fixes parameter name `return_tensor -> return_tensors` in docstring.

Fixes potential confusion.

I believe this is my biggest contribution yet 🤣 

## 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?

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
@sgugger

_The documentation is not available anymore as the PR was closed or merged._