nixos/firewall: remove stray quote from package option

This quote is not needed and shows up rendered in the manpage documenting this option.

Changed files
+1 -1
nixos
modules
services
networking
+1 -1
nixos/modules/services/networking/firewall.nix
···
example = literalExpression "pkgs.iptables-legacy";
description =
lib.mdDoc ''
-
The iptables package to use for running the firewall service."
+
The iptables package to use for running the firewall service.
'';
};