Update Configuration indicating that ws_coliposte_letter_service is required

Update configuration to indicate that `lexik_colissimo.ws_coliposte_letter_service` node is required (..because it is) fixing at the same time container exception when not set :

```
[Symfony\Component\Debug\Exception\ContextErrorException]                                                                                        
  Notice: Undefined index: ws_coliposte_letter_service in vendor/lexik/colissimo-bundle/Lexik/Bundle/Coliss  
  imoBundle/DependencyInjection/LexikColissimoExtension.php line 28
```