docs: Fix broken "Good First Issue" link in CONTRIBUTING.md

[famouswizard]

What does this PR do?

The link labeled "Good First Issue" in the documentation is broken—it currently points to a non-existent page.

I've updated it to the correct URL, which should now direct users to the appropriate issues page.
This should help contributors find suitable issues more easily.

Changes:

• Updated the "Good First Issue" link from https://github.com/huggingface/trl/contribute to https://github.com/huggingface/trl/issues.

Before submitting

• 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,
  Pull Request section?
• Was this discussed/approved via a GitHub issue? 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.
• Did you write any new necessary tests?

Who can review?

Anyone