Small Terraform 0.12 fix

Author: OliverKlette85

main.tf

@@ -39,7 +39,7 @@ resource "aws_rds_cluster" "main" {
   preferred_maintenance_window    = var.preferred_maintenance_window
   port                            = local.port
   db_subnet_group_name            = aws_db_subnet_group.main[0].name
-  vpc_security_group_ids          = [concat([aws_security_group.main[0].id], var.extra_security_groups)]
+  vpc_security_group_ids          = concat([aws_security_group.main[0].id], var.extra_security_groups)
   snapshot_identifier             = var.snapshot_identifier
   storage_encrypted               = var.storage_encrypted
   apply_immediately               = var.apply_immediately