Update region tags in appengine helloworld

Author: Shun Fan

appengine/helloworld/src/main/webapp/WEB-INF/appengine-web.xml

@@ -1,5 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- [START_EXCLUDE] -->
 <!--
   Copyright 2016 Google Inc. All Rights Reserved.
   Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,9 +11,10 @@
   See the License for the specific language governing permissions and
   limitations under the License.
 -->
-<!-- [END_EXCLUDE] -->
+<!-- [START config] -->
 <appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
   <application>YOUR-PROJECT-ID</application>
   <version>YOUR-VERSION-ID</version>
   <threadsafe>true</threadsafe>
 </appengine-web-app>
+<!-- [END config] -->