make userId final in MembershipService

functions/api/lib/membership/membership_service.dart

@@ -3,7 +3,7 @@ import 'package:lib/lib.dart';
 import 'package:point/membership/team.dart';
 
 class MembershipService extends ClientFunction {
-  String _userId;
+  final String _userId;
   MembershipService({required super.host, required super.projectId, required super.apiKey, required String userId}) :
     _userId = userId;