Add missing semi-colon

Closes: graphql#352

Author: wincent

site/graphql-js/Tutorial-Mutations.md

@@ -133,7 +133,7 @@ var root = {
     fakeDatabase[id] = input;
     return new Message(id, input);
   },
-}
+};
 
 var app = express();
 app.use('/graphql', graphqlHTTP({