problem-reports: convert a svnweb link for UPDATING to cgit.

1. Uncomment and complete these metadata fields, as appropriate: #
2. PR:
3. Submitted by:
4. Reported by:
5. Reviewed by:
6. Approved by:
7. Obtained from:
8. MFC after:
9. MFH:
10. Relnotes:
11. Security:
12. Sponsored by:
13. Pull Request:
14. Differential Revision: #
15. Description of fields to fill in above: 72 columns --|
16. PR: If and which Problem Report is related.
17. Submitted by: If someone else sent in the change.
18. Reported by: If someone else reported the issue.
19. Reviewed by: If someone else reviewed your modification.
20. Approved by: If you needed approval for this commit.
21. Obtained from: If the change is from a third party.
22. MFC after: N [day[s]|week[s]|month[s]]. Request a reminder email.
23. MFH: Ports tree branch name. Request approval for merge.
24. Relnotes: Set to 'yes' for mention in release notes.
25. Security: Vulnerability reference (one per line) or description.
26. Sponsored by: If the change was sponsored by an organization.
27. Pull Request: https://github.com/freebsd/<repo>/pull/### (*full* GitHub URL needed).
28. Differential Revision: https://reviews.freebsd.org/D### (*full* phabric URL needed).