Update the example for kubectl port-forward

Clarify that `REMOTE_PORT` is interpreted as identifying a _Service_ port when provided `TYPE` is `service`.
Also, highlight support for specifying a named port as `REMOTE_PORT`.

content/en/examples/application/guestbook/redis-master-service.yaml

@@ -8,7 +8,8 @@ metadata:
     tier: backend
 spec:
   ports:
-  - port: 6379
+  - name: redis
+    port: 6379
     targetPort: 6379
   selector:
     app: redis