Add region tags for flex storage docs (GoogleCloudPlatform#301)

Author: carieshmarie

appengine/flexible/storage/app.php

@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+# [START app]
 # [START import_client]
 use Google\Cloud\Storage\StorageClient;
 # [END import_client]
@@ -77,5 +78,6 @@
     # [END create_client]
     return $storage;
 };
+# [END app]
 
 return $app;