Fix debug config reference dump_destination

[vinceAmstoutz]

The method loadFromExtension does not exist in the Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

The only way to do this is to use the extension method, which then calls loadFromExtension , as you can see in the class when using CRTL+F :

[vinceAmstoutz]

@javiereguiluz As we discussed on Slack, I created this PR to fix the doc