Add dhcp to firewall configuration in quickstart

For some reason, adding dhcp as a firewalld service wasn't required for dhcpd;
but it appears to be required for dnsmasq.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

userdocs/quickstart/debian12.rst

@@ -50,6 +50,7 @@ to the default zone, and reload.
 
    sudo systemctl restart firewalld
    sudo firewall-cmd --permanent --add-service warewulf
+   sudo firewall-cmd --permanent --add-service dhcp
    sudo firewall-cmd --permanent --add-service nfs
    sudo firewall-cmd --permanent --add-service tftp
    sudo firewall-cmd --reload