docs: All broken links were fixed in CONTRIBUTING.md file.
# What does this PR do?
### This PR fixes the broken links in contributing file.

- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
**Yes, this PR does fix broken links in contributing file.**

- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
**Yes, I've read the contributor guideline.**

- [ ] 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.
**No, it was not discussed in any GitHub issue or in the form.**

- [x] Did you write any new necessary tests?
**Yes, before creating a PR, I've checked the broken links in the contributing file.**

_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for the fixes!

You're welcome!