fix names of environment variables

doc/ci/services/postgres.md

@@ -37,9 +37,9 @@ And then configure your application to use the database, for example:
 
 ```yaml
 Host: postgres
-User: $PG_USER
-Password: $PG_PASSWORD
-Database: $PG_DB
+User: $POSTGRES_USER
+Password: $POSTGRES_PASSWORD
+Database: $POSTGRES_DB
 ```
 
 If you're wondering why we used `postgres` for the `Host`, read more at