yaml is the correct lexer name

Hiya!

This will fix the lack of code highlighting here: http://symfony.com/doc/current/bundles/DoctrineMigrationsBundle/index.html#configuration

We're adding Travis build coverage to the official Symfony docs, which will include any of these core bundles included at symfony.com. So, merging this will be much appreciated, as it's causing Travis to fail currently :).

Thanks!