Update zone.xml.erb

The syntax for  'firewall-cmd --zone=$zone --add-source=[address/mask]' requires you add lines of the form '<source address="[address/mask]" />' to the zone.xml file under /etc/firewalld/zones/. 
This patch makes firewalld understand the zone.xml file as intended.
Merge pull request #2 from RamsesRodenburg/patch-1

Update zone.xml.erb