Fix Prometheus help link to lead to proper Prometheus help page

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

@@ -322,7 +322,7 @@
       \. This setting requires a
       = link_to 'restart', help_page_path('administration/restart_gitlab')
       to take effect.
-      = link_to icon('question-circle'), help_page_path('administration/monitoring/performance/introduction')
+      = link_to icon('question-circle'), help_page_path('administration/monitoring/prometheus/index')
     .form-group
       .col-sm-offset-2.col-sm-10
         .checkbox