Another missing --project=$PROJECT

This isn't required to be there if a user sets their project correctly but is helpful for folks running partial steps from this guide.

xpk-large-scale-guide.sh

@@ -101,6 +101,7 @@ gcloud compute routers create "${ROUTER_NAME}" \
 gcloud compute routers nats create "${NAT_CONFIG}" \
   --router="${ROUTER_NAME}" \
   --region="${REGION}" \
+  --project="${PROJECT}" \
   --auto-allocate-nat-external-ips \
   --nat-all-subnet-ip-ranges \
   --enable-logging