Alter redis cache subnet group

Using this answer from StackOverflow: https://stackoverflow.com/questions/59643874/aws-cdk-error-when-deploying-redis-elasticache-subnet-group-belongs-to-a-diffe

mentor_match_infra/mentor_match_infra/mentor_match_stack.py

@@ -58,7 +58,7 @@ def __init__(self, scope: Construct, id_: str, vpc: ec2.Vpc):
             engine="redis",
             cache_node_type="cache.t2.micro",
             num_cache_nodes=1,
-            cache_subnet_group_name=redis_subnet_group.cache_subnet_group_name,
+            cache_subnet_group_name=redis_subnet_group.ref,
             vpc_security_group_ids=[self._redis_sec_group.security_group_id],
         )