[Components][Routing] Fix addPrefix() sample code

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

The sample code used for demonstrating the
[`Symfony\Component\Routing\RouteCollection::addPrefix()`](http://api.symfony.com/2.3/Symfony/Component/Routing/RouteCollection.html#method_addPrefix) method did not match the signature. I've updated the code to show the provided features when adding a nested route collection. + some minor fixes along the way.