Update mutations.rst

I believe the `[1]` was ommitted from the `from_global_id` call as that method returns a tuple of type and id, of which we're only interested in the id here. Took me half a day to figure out why this code wasn't working today. See function def here: https://github.com/graphql-python/graphql-relay-py/blob/master/graphql_relay/node/node.py#L67

Author: sweenr

docs/mutations.rst

@@ -214,7 +214,7 @@ You can use relay with mutations. A Relay mutation must inherit from
 
         @classmethod
         def mutate_and_get_payload(cls, root, info, text, id):
-            question = Question.objects.get(pk=from_global_id(id))
+            question = Question.objects.get(pk=from_global_id(id)[1])
             question.text = text
             question.save()
             return QuestionMutation(question=question)
@@ -226,4 +226,4 @@ Relay ClientIDMutation accept a ``clientIDMutation`` argument.
 This argument is also sent back to the client with the mutation result
 (you do not have to do anything). For services that manage
 a pool of many GraphQL requests in bulk, the ``clientIDMutation``
-allows you to match up a specific mutation with the response.
+allows you to match up a specific mutation with the response.