Use singular namespaces

[BafS]

The demo has singular namespaces except for DataFixtures, Events and Migrations.

The documentation uses singular form, for example App\Event\... (https://symfony.com/doc/current/event_dispatcher.html), I think it would be good to be in-pair with the doc and the conventions.

What do you think ?

[javiereguiluz]

Yes ... using Event in singular would be in sync with the docs (the other namespaces in plural are OK and we can keep them). Would you be able to make a pull request to fix this? Thanks!

[BafS]

@javiereguiluz what is the rational behind the singularization of namespaces except DataFixtures? Why do we have this exception?