Change comment in ndb snippets to object property (GoogleCloudPlatform#1150)

Before this change, the documentation indicated that key property would
be on the Account class instead of the account object. This changes that
to properly reflect the key property is on the object.

Author: howinator

appengine/standard/ndb/entities/snippets.py

@@ -118,7 +118,7 @@ def create_entity_with_generated_id():
     # note: no id kwarg
     account = Account(username='Sandy', userid=1234, email='[email protected]')
     account.put()
-    # Account.key will now have a key of the form: ndb.Key(Account, 71321839)
+    # account.key will now have a key of the form: ndb.Key(Account, 71321839)
     # where the value 71321839 was generated by Datastore for us.
     return account