correct the prometheus feature configuration according to...

correct the prometheus feature configuration according to https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-config-template/gitlab.rb.template#L1147

doc/administration/monitoring/performance/prometheus.md

@@ -30,7 +30,7 @@ To enable Prometheus:
 1. Find and uncomment the following line, making sure it's set to `true`:
 
     ```ruby
-    node_exporter['enable'] = true
+    prometheus['enable'] = true
     ```
 
 1. Save the file and [reconfigure GitLab][reconfigure] for the changes to