Update routing.yml to use path instead of deprecated pattern

According to Symfony, "path" should be used, not "pattern" in routing.yml.

"The path option was introduced in Symfony 2.2, pattern is used in older versions."
http://symfony.com/doc/2.3/book/routing.html

Currently both path and pattern are supported. But pattern will be fully deleted from Symfony in v3.
https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md#routing

So in order to prevent that issue for all extensions, we should promote the use of "path" now instead, and since this extension serves as a demo for authors... ;)