Fix incorrect example in README

[revov]

According to https://github.com/davglass/license-checker/blob/master/lib/index.js#L264 the first parameter passed to the init callback is the error and the second one is the actual return value. This was incorrectly specified in the readme and is addressed in this pull request.

[davglass]

Thanks! I blame @bengl for missing that one :)

[bengl]

Oops!