docu: without specifying the asn to be 16550 this example gives an error (#3706)

Error:
com.google.apps.framework.request.StatusException: <eye3 title='INVALID_ARGUMENT'/> generic::INVALID_ARGUMENT: Invalid value for field 'resource.router': 'projects/<PREFIX>-prod-net-core-0/regions/europe-north1/routers/encrypted-vlan-attachment-b-underlay'. The Cloud Router used by PARTNER type interconnect attachments must be assigned a local ASN of '16550'

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>

modules/net-vlan-attachment/README.md

@@ -613,6 +613,7 @@ module "example-va-a" {
   description = "example-va-a vlan attachment"
   peer_asn    = "65001"
   router_config = {
+    asn    = 16550
     create = true
   }
   partner_interconnect_config = {
@@ -630,6 +631,7 @@ module "example-va-b" {
   description = "example-va-b vlan attachment"
   peer_asn    = "65001"
   router_config = {
+    asn    = 16550
     create = true
   }
   partner_interconnect_config = {