docs: update vich example to use the Attribute namespace instead

[cngJo]

The "old" namespace works, but is deprecated since v2 of the bundle and throws deprecation messages in the logs.

e.G. in the output of the PHPUnit tests I get when following the docs:

2) /var/www/html/vendor/vich/uploader-bundle/src/Mapping/Annotation/Uploadable.php:24
Since vich/uploader-bundle 2.9: The "Vich\UploaderBundle\Mapping\Annotation\Uploadable" class is deprecated, use "Vich\UploaderBundle\Mapping\Attribute\Uploadable" instead.

[soyuka]

thanks!