Arr::first not updated after Laravel 5.3 migration

https://github.com/dingo/api/blob/5fa3c344f511e6ee1c9c0de42815978e1ed837cc/src/Console/Command/Docs.php#L207

There appears to be an issue on this line.

According to the [Laravel 5.3 migration guide](https://laravel.com/docs/5.3/upgrade) the Arr "Key / Value Order Change" should change.

> In previous versions of Laravel, the $key was passed first. Since most use cases are only interested in the $value it is now passed first. You should do a "global find" in your application for these methods to verify that you are expecting the $value to be passed as the first argument to your Closure.