Update reference.md to fix broken link

[toniprice]

Update link from lab.github.com to skills.github.com
Closes #167

[github-actions]

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Rendered Changes

🔍 Inspect the changes: https://github.com/LibraryCarpentry/lc-git/compare/md-outputs..md-outputs-PR-184

The following changes were observed in the rendered markdown documents:

 md5sum.txt   | 2 +-
 reference.md | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

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⏱️ Updated at 2025-08-06 08:35:20 +0000

[alex-ball]

@toniprice , thank you so much for your contribution. With unfortunate synchronicity, we had two pull requests addressing issue #167 at almost the same time. Since we've merged the other one, it would be great if this pull request could focus solely on correcting the pymarc GitLab link. I've put what I think is the right link in a comment.

Please could you update your pull request with change to the link. (I think Git will handle/ignore the GitHub Skills change if you leave it in.)

Thanks again!

[alex-ball]

@toniprice, thanks again for your contribution.

I'll close this pull request now as the changes have been merged from #183 and #185 instead.