Fix 500 on appearance page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/admin/appearances/_form.html.haml

@@ -13,7 +13,7 @@
     .col-sm-10
       = f.text_area :description, class: "form-control", rows: 10
       .hint
-        Description parsed with #{link_to "GitLab Flavored Markdown", help_markdown_path, target: '_blank'}.
+        Description parsed with #{link_to "GitLab Flavored Markdown", help_page_path('markdown', 'markdown'), target: '_blank'}.
   .form-group
     = f.label :logo, class: 'control-label'
     .col-sm-10