📝(doc) Use an empty directory for postgres

Use an empty directory for postgres data, otherwise it will complain the
directory is not empty and fail to start.

docs/examples/compose/compose.yaml

@@ -10,7 +10,7 @@ services:
     - env.d/postgresql
     - env.d/common
     volumes:
-    - ./data/databases/backend:/var/lib/postgresql/data/pgdata
+    - ./data/databases/backend:/var/lib/postgresql/data
 
   redis:
     image: redis:5