Update ngfw.md to include link_id

Referenced the updated TF as mentioned in the Slack thread here: https://panw-rnd.slack.com/archives/C0315M3EFD3/p1715893902779599?thread_ts=1706553852.909279&cid=C0315M3EFD3

docs/resources/ngfw.md

@@ -25,7 +25,7 @@ resource "cloudngfwaws_ngfw" "example" {
   vpc_id      = aws_vpc.example.id
   account_id  = "12345678"
   description = "Example description"
-
+  link_id = "Link-81e80ccc-357a-4e4e-8325-1ed1d830cba5"
   endpoint_mode = "ServiceManaged"
   subnet_mapping {
     subnet_id = aws_subnet.subnet1.id