fix broken link

howto/trac.md

@@ -55,6 +55,8 @@ CI). Those services *might* eventually be migrated to GitLab, but
 that's not part of the current migration plan. See [issue 36][] for
 the followup on that.
 
+[issue 36]: https://gitlab.torproject.org/tpo/tpa/gitlab/-/issues/36
+
 Again, the canonical copy for source code hosted by git is:
 
  * `git-rw.torproject` - writable git repositories over SSH