Update phpunit command

[sasensi]

Command was not working with a just cloned demo application.
Found solution on this page: http://symfony.com/doc/current/components/phpunit_bridge.html

[javiereguiluz]

@sasensi thanks a lot for this contribution and for your help to improve Symfony Docs. However, this is considered correct. We recently updated (see https://github.com/symfony/symfony-docs/pull/9452/files) to update for the latest practices of Symfony.

You are right that the Symfony Demo app doesn't work like explained in the docs, so I've opened an issue there (see symfony/demo#835) so we can fix it.

Let's close this issue for now and let's work on the Symfony Demo repository to see if we can fix it. Thanks!