Fix broken links
Author: kurianbenoy
Pull request: yes

# What does this PR do?

- Fix broken links in page: https://huggingface.co/docs/transformers/model_doc/marian

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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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Models:

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- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Have updated the documentation. Thanks for catch @LysandreJik