Fix `doGenerate()` signature

This router does not work because of a commit introduced in Symfony
`2.4.1`. It looks like a BC break but the change has been made on a
"protected" method, so it is not a BC break...

cf.  https://github.com/symfony/symfony/commit/557dfaa2c1623e2160d75473a8a736a1d9522f6b#commitcomment-4988320