chore: fix mismatched region tag (GoogleCloudPlatform#4259)

Co-authored-by: Leah E. Cole <[email protected]>

Author: Ace Nassri

appengine/standard/images/guestbook/main.py

@@ -124,7 +124,7 @@ def post(self):
         # [START sign_handler_2]
         greeting.avatar = avatar
         greeting.put()
-        # [END sign_handler_1]
+        # [END sign_handler_2]
 
         self.redirect('/?' + urllib.urlencode(
             {'guestbook_name': guestbook_name}))