fix: update simple IPv6 subnet example (#498)

Author: kanya-approve

examples/simple_ipv6_project/main.tf

@@ -29,11 +29,11 @@ module "test-vpc-module" {
 
   subnets = [
     {
-      subnet_name   = "subnet-01"
-      subnet_ip     = "10.10.10.0/24"
-      subnet_region = "us-west1"
-      stack         = "IPV4_IPV6"
-      ipv6_type     = "EXTERNAL"
+      subnet_name      = "subnet-01"
+      subnet_ip        = "10.10.10.0/24"
+      subnet_region    = "us-west1"
+      stack_type       = "IPV4_IPV6"
+      ipv6_access_type = "EXTERNAL"
     },
     {
       subnet_name           = "subnet-02"