Update execution doc with correct way to use variables (#920)

* Fix issue #890

* Change to `root`

Author: jkimbo

docs/execution/execute.rst

@@ -24,7 +24,7 @@ You can pass context to a query via ``context``.
     class Query(graphene.ObjectType):
         name = graphene.String()
 
-        def resolve_name(self, info):
+        def resolve_name(root, info):
             return info.context.get('name')
 
     schema = graphene.Schema(Query)
@@ -33,18 +33,18 @@ You can pass context to a query via ``context``.
 
 
 Variables
-_______
+_________
 
 You can pass variables to a query via ``variables``.
 
 
 .. code:: python
 
     class Query(graphene.ObjectType):
-        user = graphene.Field(User)
+        user = graphene.Field(User, id=graphene.ID(required=True))
 
-        def resolve_user(self, info):
-            return info.context.get('user')
+        def resolve_user(root, info, id):
+            return get_user_by_id(id)
 
     schema = graphene.Schema(Query)
     result = schema.execute(