Fix variable typo in validation task

roles/custom/matrix-client-element/tasks/validate_config.yml

@@ -35,7 +35,7 @@
           You need to define a required configuration setting (`{{ item }}`).
       when: "vars[item] == ''"
       with_items:
-        - matrix_synapse_admin_container_labels_traefik_hostname
+        - matrix_client_element_container_labels_traefik_hostname
         - matrix_client_element_container_labels_traefik_path_prefix
 
     # We ensure it doesn't end with a slash, because we handle both (slash and no-slash).