Fix example of apollo-server-express usage

Update valid usage of apollo-server-express package from package's latest docs:
https://github.com/apollographql/apollo-server/blob/master/packages/apollo-server-express/README.md

docs/source/express.md

@@ -10,7 +10,7 @@ For example: if your server is already running on port 3000 and accepts GraphQL
 ```js
 import express from 'express';
 import bodyParser from 'body-parser';
-import { graphqlExpress } from 'apollo-server-express';
+import { ApolloServer, gql }  from 'apollo-server-express';
 import { createServer } from 'http';
 import { execute, subscribe } from 'graphql';
 import { PubSub } from 'graphql-subscriptions';
@@ -20,7 +20,10 @@ import { myGraphQLSchema } from './my-schema';
 const PORT = 3000;
 const app = express();
 
-app.use('/graphql', bodyParser.json(), graphqlExpress({ schema: myGraphQLSchema }));
+app.use('/graphql', bodyParser.json());
+
+const apolloServer = new ApolloServer({ schema: myGraphQLSchema });
+apolloServer.applyMiddleware({ app });
 
 const pubsub = new PubSub();
 const server = createServer(app);