Fix link pointing to firewall.conf in the documentation
Originally created by @elouann on #10855 (Redmine)
In https://tails.boum.org/contribute/design/Tor_enforcement/
the link to etc/firewall.conf is broken.
I guess the right link is etc/ferm/ferm.conf, please confirm and
eventually assign this ticket to me, I’ll submit a patch.
I will also fix the link to 000-resolv-conf
Feature Branch: elouann:documentation