Fix capitalization: postgres → Postgres

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

roles/custom/matrix_playbook_migration/tasks/validate_config.yml

@@ -423,7 +423,7 @@
 - name: (Deprecation) Catch and report devture_postgres variables
   ansible.builtin.fail:
     msg: |-
-      The postgres role in the playbook now lives under the MASH organization (https://github.com/mother-of-all-self-hosting/ansible-role-postgres).
+      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_*` -> `postgres_*`).