update to fix proper connection string for postgres

Author: nullfunc

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

@@ -50,7 +50,7 @@ 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_PASSWORD}@database:5432/postgres
+      CONNECTURL: postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@database:5432/postgres
   database:
     image: postgres:15
     x-defang-postgres: true