Update xFormers MinGPT notebook reference

[emmanuel-ferdman]

What does this PR do?

Small PR - updates the xFormers MinGPT Colab notebook reference.

Before submitting

• [x] Did you have fun?
  • Make sure you had fun coding 🙃
• [x] Did you read the contributor guideline?
• [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
  • [ ] N/A
• [x] Did you make sure to update the docs?
  • [ ] N/A
• [ ] Did you write any new necessary tests?
  • [ ] N/A
• [ ] Did you update the changelog? (if needed)
  • [ ] N/A

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

[emmanuel-ferdman]

@danthe3rd, @lw, @zyan0 can you please review this PR?