Update validate_config.yml for migration: adopt a common instruction

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -320,7 +320,7 @@
         msg: >-
           Your configuration contains a variable, which now has a different name.
 
-          Please rename all variables (`matrix_bot_postmoogle_` -> `matrix_postmoogle_`) on your vars.yml.
+          Please rename all variables (`matrix_bot_postmoogle_` -> `matrix_postmoogle_`) on your configuration file (vars.yml).
 
           The following variables in your configuration need to be renamed: {{ matrix_playbook_migration_postmoogle_migration_vars.keys() | join(', ') }}
       when: "matrix_playbook_migration_postmoogle_migration_vars | length > 0"