Fix version link on help page for pre-release versions
After https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22015 we started linking to the tag of the current version.
This will obviously not be correct for pre-release versions, as that tag is created on the first commit of the cycle, not the currently deployed one. The link for pre-release versions would also result in a 404, as we don't name the tag exactly the same as we do in the VERSION
file.
/cc @stanhu