Use correct variable name in gitlab.yml.erb

files/gitlab-cookbooks/gitlab/templates/default/gitlab.yml.erb

@@ -43,7 +43,7 @@ production: &base
     ## User settings
     default_projects_limit: <%= @gitlab_default_projects_limit %>
     default_can_create_group: <%= @gitlab_default_can_create_group %>  # default: true
-    username_changing_enabled: <%= @username_changing_enabled %> # default: true - User can change her username/namespace
+    username_changing_enabled: <%= @gitlab_username_changing_enabled %> # default: true - User can change her username/namespace
     ## Default theme
     ##   BASIC  = 1
     ##   MARS   = 2