Fix link to the TPU launcher script in the pytorch examples
Author: amineabdaoui
Pull request: yes

# What does this PR do?

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Hi @sgugger, @patil-suraj,

The link to the TPU launcher script in the pytorch examples is broken.

Thanks

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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