Fix link to gitlab-markup in the user markdown documentation
Currently in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md#L29, the link in
dependency to do so. Please see the [github-markup gem readme](https://github.com/gitlabhq/markup#markups) for more information
should point to https://gitlab.com/gitlab-org/gitlab-markup
Referenced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22122#note_106667185