Update Laravel example to work when configuration is cached

[Krisell]

The Laravel example of reading the application key doesn't work in a production setting where the configuration cache is used. All calls to env() return null then and config() should be used instead.

Using config('app.key') always works, cached or not. Of course a Laravel developer already knows this so the risk of it causing an actual problem is very low, but I think any Laravel example should follow Laravel conventions.

Laravel documentation about this https://laravel.com/docs/8.x/deployment#optimizing-configuration-loading

[Sephster]

Thanks for this @Krisell - happy to merge