Update hardcoded Youtube channel url prefix

Hi!

I detected that the user should insert "@" before to text the user name.

So I suggest to add "@" to the beggining. I'm attaching this patch, hope it helps!

Hi again!

I've just request a new merge request to this issue fork.

Please review it and if everything ok, it could be merged :).

Thank you!

Automatically closed - issue fixed for 2 weeks with no activity.