Fixed incorrect reference to RDS subnet group

rds/main.tf

@@ -13,7 +13,7 @@ resource "aws_db_instance" "postgresql" {
     // Because we're assuming a VPC, we use this option, but only one SG id
     vpc_security_group_ids = ["${var.security_group_ids}"]
     // We're creating a subnet group in the module and passing in the name
-    db_subnet_group_name = "${var.stack_name}-db-subnet-group"
+    db_subnet_group_name = "${aws_db_subnet_group.postgresql.id}"
     parameter_group_name = "default.postgres9.4"
     // We want the multi-az setting to be toggleable, but off by default
     multi_az = "${var.rds_is_multi_az}"