nixos/firewall: fix documentation link formatting

Changed files
+1 -1
nixos
modules
services
networking
+1 -1
nixos/modules/services/networking/firewall.nix
···
Loading of helpers is recommended to be done through the
CT target. More info:
-
https://home.regit.org/netfilter-en/secure-use-of-helpers/
+
<https://home.regit.org/netfilter-en/secure-use-of-helpers/>
'';
};