Use the Request factory for multisite

https://github.com/symfony/symfony/pull/8957 breaks the current way to override the `Request` class explained in the doc if subrequests are used.

This PR fix the doc to use the new factory system.

https://github.com/symfony/symfony/pull/11040 (if merged) will revert the Symfony BC break but using the factory is the new recommended way to extend the `Request` class.

cc @Sinjonathan