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/el.rst

@@ -49,6 +49,7 @@ to the default zone, and reload.
 
    systemctl restart firewalld
    firewall-cmd --permanent --add-service=warewulf
+   firewall-cmd --permanent --add-service=dhcp
    firewall-cmd --permanent --add-service=nfs
    firewall-cmd --permanent --add-service=tftp
    firewall-cmd --reload