[Cookbook][Configuration] Fix front controller sample codes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2+
| Fixed tickets | -

* The code examples for the `prod` and `benchmark` front controller were still using 2.0 code. 
* Before the `benchmark` front controller it says *Copy the `web/app.php` ...* but then the code differs from the `app.php` not only in the enironment key.

I thought about removing the comments about caching to keep the code short. But then you would maybe wonder about the `$loader` variable declaration. Morover, the text says *If you open up one of these files ...*, so maybe it's better to show the file as is when you open it.