Correct typographical error in README.md
Author: Seyviour
Pull request: yes

Corrected a typo ('Downlowd' to 'Download')

# What does this PR do?

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Fixes # (issue)


## 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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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
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