fixed gitlab link (#570)

Co-authored-by: Jakob Lenfers <lenfers@bremen-social-sciences.de>

layouts/_default/single.html

@@ -136,7 +136,7 @@
           {{ end }}
           <div class="col-md-6 btn-improve-page">
             {{ if ( eq site.Params.GitForge "gitlab" ) }}
-              <a href="{{ site.Params.GitRepo }}/-/edit/{{ .Scratch.Get "GitBranch" }}/{{ .File.Path }}" title="{{ i18n "improve_this_page" }}" target="_blank" rel="noopener">
+              <a href="{{ site.Params.GitRepo }}/-/edit/{{ .Scratch.Get "GitBranch" }}/content/{{ .File.Path }}" title="{{ i18n "improve_this_page" }}" target="_blank" rel="noopener">
             {{ else if ( eq site.Params.GitForge "gitea" ) }}
               <a href="{{ site.Params.GitRepo }}/_edit/{{ .Scratch.Get "GitBranch" }}/content/{{ .File.Path }}" title="{{ i18n "improve_this_page" }}" target="_blank" rel="noopener">
             {{ else }} <!--- Make Github-style the default -->