Fix error in grant code example

[esarafianou]

This PR updates the code sample of README.md regarding granting code. The scope is available only in the areq object, not the ares.

Here's the return object of the request function, which includes the scope: https://github.com/jaredhanson/oauth2orize/blob/master/lib/grant/code.js#L115

[coveralls]

Coverage remained the same at 99.789% when pulling 98b73841cef9f3eef6961ec3e4d492b6ae130af6 on esarafianou:readme_grantcode into 07b1a31005bbb1c005fc2865c83d8a61f4016997 on jaredhanson:master.