NumberFormatEvent should not inherit from GenericEvent class

This is a really tiny and unimportant one:

Just saw accidentially that \Drupal\commerce_price\Event\NumberFormatEvent is inheriting from \Symfony\Component\EventDispatcher\GenericEvent instead of just inhering from \Symfony\Component\EventDispatcher\Event, like all other events in Commerce. The generic event offers a "subject" property and an "arguments" property inlcuding getters and array access and iterator implementation. However, we're overloading the constructor anyway, so these properties never will be set. Based on that and the fact, that no other event in Commerce uses the generic base class, I'm assuming this was just a careless PhpStorm autocomplete mistake ;)