[WCM] reflect the changes in the directory implied by the switch to PSR-4 compatible autoloading

When the autoloading is switched from PSR-0 to PSR-4 which removes the target directory (see symfony-cmf/symfony-cmf#185 and symfony-cmf/CreateBundle#110), the directory structure in a CMF project's vendor directory changes. This pull request reflects the changes in the documentation.
do we need some `.. since::` here maybe?