Update DomCrawler example

[javiereguiluz]

According to symfony/symfony#18660 (comment) some example of the DomCrawler docs is wrong.

This example is no longer valid ;)

use Symfony\Component\CssSelector\CssSelector;

CssSelector::disableHtmlExtension();
$crawler = $crawler->filter('default|entry media|group yt|aspectRatio');

[xabbuh]

This should be done for the 2.8 branch as this is the version where the new CssSelector API was introduced.

[wouterj]

This is fixed by #6536