Update ExternalName example

As per design proposal: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/service-external-name.md

The existing example is not validated by Kubernetes 1.6.3

docs/concepts/services-networking/service.md

@@ -140,6 +140,8 @@ metadata:
 spec:
   type: ExternalName
   externalName: my.database.example.com
+  ports:
+  - port: 12345
 ```
 
 When looking up the host `my-service.prod.svc.CLUSTER`, the cluster DNS service