Correction in documentation about importing express-graphql. (graphql#153)

Author: kalinchernev

site/graphql-js/APIReference-ExpressGraphQL.md

@@ -11,7 +11,7 @@ The `express-graphql` module provides a simple way to create an [Express](https:
 
 ```js
 import graphqlHTTP from 'express-graphql'; // ES6
-var graphqlHTTP = require('graphql'); // CommonJS
+var graphqlHTTP = require('express-graphql'); // CommonJS
 ```
 
 ### graphqlHTTP