Improved the example to generate URLs in the console

[javiereguiluz]

This fixes #8827 and a minor issue (code was tabulated with 3 white spaces).

When merging in 4.0, the code must be tweaked to use $this->router variable instead of $router.

[Starojitski]

Thanks for such a quick action - having the $router variable makes what is going on much clearer!
I have a feeling that generating url in console is often used when sending e-mails. There are two use cases here:

• Those emails have links for various actions email recipients can perform by clicking on the links within the message body - such as confirm account creation, etc. Your example shows how to generate URL for a named route.
• Adding images/attachments to the emails. Ideally the examples would also show how to generate url for assets as well. This is what i needed. But I have limited experience with Symfony and quite possible i am barking at the wrong tree here and the solution is very easy or must be found in some other place