fix: added count index for associating nat gateway (#5)

vpc.tf

@@ -94,7 +94,7 @@ resource "aws_route" "ngw_route" {
   count                  = var.create_private_subnets || length(var.nat_type) > 0 ? var.number_of_aws_az_use : 0
   destination_cidr_block = "0.0.0.0/0"
   network_interface_id   = var.nat_type == "instance" ? module.ec2_nat_instance.0.network_interface_id : null
-  nat_gateway_id         = var.nat_type == "gateway" ? element(aws_nat_gateway.ngw.*.id, var.number_of_nat_gw) : null
+  nat_gateway_id         = var.nat_type == "gateway" ? element(aws_nat_gateway.ngw.*.id, count.index) : null
   route_table_id         = element(aws_route_table.private_route_table.*.id, count.index)
 }