Update managed-postgres.md: fix interpolated CONNECTURL

Author: lionello

docs/concepts/managed-storage/managed-postgres.md

@@ -53,7 +53,7 @@ You can connect to the managed Postgres instance using the name of your service
       // highlight-end
       # Note: you can create a connection string by using interpolation,
       # reference config variables by using ${<config name>}
-      CONNECTURL: postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@database:5432/${POSTGRES_DB}
+      CONNECTURL: postgresql://postgres:${POSTGRES_PASSWORD}@database:5432/postgres
   database:
     image: postgres:15
     x-defang-postgres: true