Fix validate_config.yml

`devture_postgres_*` -> `postgres_*`

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -380,7 +380,7 @@
       The postgres role in the playbook now lives under the MASH organization (https://github.com/mother-of-all-self-hosting/ansible-role-postgres).
       The new role is pretty much the same, but uses differently named variables.
 
-      Please change your configuration (vars.yml) to rename all `devture_postgres_`-prefixed variables (`devture_postgres_*` -> `devture_postgres_*`).
+      Please change your configuration (vars.yml) to rename all `devture_postgres_`-prefixed variables (`devture_postgres_*` -> `postgres_*`).
 
       The following variables in your configuration need to be renamed: {{ vars | dict2items | selectattr('key', 'match', 'devture_postgres_.*') | map (attribute='key') | join(', ') }}
   when: "vars | dict2items | selectattr('key', 'match', 'devture_postgres_.*') | list | items2dict"