Fix missing argument error in graphene example (graphql#198)

TypeError: resolve_hello() takes 3 positional arguments but 4 were given

Author: alibitek

site/code/index.html.js

@@ -185,7 +185,7 @@ import graphene
 class Query(graphene.ObjectType):
   hello = graphene.String()
 
-  def resolve_hello(self, args, info):
+  def resolve_hello(self, args, context, info):
     return 'Hello world!'
 
 schema = graphene.Schema(query=Query)