Fix typo

graphql#86 fixed a typo but no CLA so I figured I'd just make a separate PR

Author: Kevin Lacker

site/graphql-js/Guides-ConstructingTypes.md

@@ -95,7 +95,7 @@ var queryType = new graphql.GraphQLObjectType({
         id: { type: graphql.GraphQLString }
       },
       resolve: function (_, {id}) {
-        return fakeDatabase[args.id];
+        return fakeDatabase[id];
       }
     }
   }