fix: correct postgres 18rc1 image name

Author: Kumzy

src/pytest_databases/docker/postgres.py

@@ -254,7 +254,7 @@ def postgres_18_service(
 ) -> Generator[PostgresService, None, None]:
     with _provide_postgres_service(
         docker_service,
-        image="postgres:18-rc1",
+        image="postgres:18rc1",
         name="postgres-18",
         xdist_postgres_isolate=xdist_postgres_isolation_level,
         host=postgres_host,