[Book][Routing] Fixed typo on PHP version of a route definition

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | -


On http://symfony.com/doc/master/book/routing.html#advanced-routing-example, the PHP version has a wrong route name.