Fix command of doc tests for local testing
Author: oneraghavan
Pull request: yes

# What does this PR do?

In the utils/prepare_for_doc_test.py file, the command to test the doc test locally had typo, this fixes it

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Fixes # (issue)


## Before submitting
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hey! You should update line 39 in a similar fashion :)

> Hey! You should update line 39 in a similar fashion :)

Yes, realised bit later, Done

@ydshieh Can we close this ? 
@LysandreJik  Please point to a next good bug to pick up.

@oneraghavan, thanks for wanting to contribute! There are a lot of issues available [here](https://github.com/huggingface/transformers/issues). Feel free to take a look and find one you'd like to try your hand at!