Fixed typo in gitlab.yml.example

Created by: mlitwiniuk

When installing gitlab on next host I've noticed a typo. So here's fix.

app/controllers/profiles_controller.rb

@@ -51,7 +51,7 @@ class ProfilesController < ApplicationController
   end
   def update_username
-    if Gitlab.config.gitlab.username_changing_enabled
+    if @user.can_change_username?
       @user.update_attributes(username: params[:user][:username])
     end