motd: Fix Questions List address

Following mailing lists migration from Mailman to Mlmmj.

https://lists.FreeBSD.org/mailman/listinfo/freebsd-questions/
-->
https://lists.freebsd.org/subscription/freebsd-questions

Adding a short link (as we already have for Faq and Handbook), so all
links can be indented the same way.

Reviewed by: imp (earlier version)
Approved by: lwhsu
Differential Revision: https://reviews.freebsd.org/D33101