[Doctrine] add missing mapping type (PHP Attributes)

[xthiago]

Support for Doctrine ORM 2.9 AttributeDrive was added a time ago, but the docs does not mention that possibility yet.

doctrine:
    orm:
        mappings:
            Your\Namespace:
                type: attribute # <-- HERE
                dir: '%kernel.project_dir%/src/Your/Namespace'
                prefix: 'Your\Namespace'
                alias: YN

[javiereguiluz]

Thiago, thanks for improving this! Congrats on your first Symfony Docs contribution 🎉