Fix import of express-graphql (graphql#899)

Author: RitikDua

site/graphql-js/Tutorial-Authentication.md

@@ -15,7 +15,7 @@ For example, let's say we wanted our server to log the IP address of every reque
 
 ```javascript
 var express = require('express');
-var graphqlHTTP = require('express-graphql');
+var { graphqlHTTP } = require('express-graphql');
 var { buildSchema } = require('graphql');
 
 var schema = buildSchema(`

site/graphql-js/Tutorial-BasicTypes.md

@@ -18,7 +18,7 @@ Each of these types maps straightforwardly to JavaScript, so you can just return
 
 ```javascript
 var express = require('express');
-var graphqlHTTP = require('express-graphql');
+var { graphqlHTTP } = require('express-graphql');
 var { buildSchema } = require('graphql');
 
 // Construct a schema, using GraphQL schema language

site/graphql-js/Tutorial-Mutations.md

@@ -73,7 +73,7 @@ Here's some runnable code that implements this schema, keeping the data in memor
 
 ```javascript
 var express = require('express');
-var graphqlHTTP = require('express-graphql');
+var { graphqlHTTP } = require('express-graphql');
 var { buildSchema } = require('graphql');
 
 // Construct a schema, using GraphQL schema language

site/graphql-js/Tutorial-ObjectTypes.md

@@ -74,7 +74,7 @@ Putting this all together, here is some sample code that runs a server with this
 
 ```javascript
 var express = require('express');
-var graphqlHTTP = require('express-graphql');
+var { graphqlHTTP } = require('express-graphql');
 var { buildSchema } = require('graphql');
 
 // Construct a schema, using GraphQL schema language