Fixes syntax error in Express example

[cujarrett]

What

The docs as they were result in an error in the Express example.

app.use("/graphql", graphqlHTTP({
                    ^

TypeError: graphqlHTTP is not a function
    at Object.<anonymous> (/Users/matt-jarrett/Development/learning-graphql/examples/express.js:14:21)

I used ES2015's destructuring to pull out graphqlHTTP and the Express example works fine.