Fix docs for pages on the separate server

`gitlab_rails['pages_enabled']=false`
actually disables UI for pages in GitLab
it was added here as a fix for NGINX:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41657/diffs

But the right way to do it is to just disable pages NGINX directly

doc/administration/pages/index.md

@@ -597,7 +597,7 @@ database encryption. Proceed with caution.
    ```ruby
    pages_external_url "http://<pages_server_URL>"
    gitlab_pages['enable'] = false
-   gitlab_rails['pages_enabled']=false
+   pages_nginx['enable'] = false
    gitlab_rails['pages_path'] = "/mnt/pages"
    ```