Fix broken link
# What does this PR do?

Fixes a broken link in model_doc/pegasus

Link was pointing to: https://github.com/huggingface/transformers/blob/master/examples/seq2seq/finetune_pegasus_xsum.sh

File had been moved to: https://github.com/huggingface/transformers/blob/master/examples/research_projects/seq2seq-distillation/finetune_pegasus_xsum.sh

Fixes # (issue)
[#9257](https://github.com/huggingface/transformers/issues/9257)

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [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?


## Who can review?

Anyone