docs: update the postgres bind mount in the compose example (#1761)

Postgres 18 moved its declared volume to /var/lib/postgresql, so the
example container fails to start with the old path.

Signed-off-by: vshulcz <vshulcz@gmail.com>

Author: vshulcz

.examples/docker-compose-postgres-storage/compose.yaml

@@ -2,7 +2,9 @@ services:
   postgres:
     image: postgres
     volumes:
-      - ./data/db:/var/lib/postgresql/data
+      # PostgreSQL 18 and above declare the volume at /var/lib/postgresql.
+      # For 17 and earlier, mount ./data/db at /var/lib/postgresql/data instead.
+      - ./data/db:/var/lib/postgresql
     ports:
       - "5432:5432"
     environment: