GitLab: restore link target

The original link became a 404 after a GitLab upgrade, which was
a problem. 2a2089c5 fixed the 404,
although by making the link point to a different page than intended.

wiki/src/contribute/working_together/GitLab.mdwn

@@ -8,7 +8,7 @@ This workflow is not set in stone: it will evolve based on what
 we learn while using GitLab.
 
 For general GitLab usage information, see the
-[[!tails_gitlab help desc="GitLab user documentation"]].
+[[!tails_gitlab help/user/index.md desc="GitLab user documentation"]].
 
 [[!toc levels=2]]