docs: fix missing forward slash

   Should like below:
    <interface type='server'>
      <mac address='52:54:00:22:c9:42'/>
      <source address='192.168.0.1' port='5558'/>
    </interface>
    ...
    <interface type='client'>
      <mac address='52:54:00:8b:c9:51'/>
      <source address='192.168.0.1' port='5558'/>
    </interface>

docs/formatdomain.html.in

@@ -3701,12 +3701,12 @@
   ...
   <devices>
     <interface type='server'>
-      <mac address='52:54:00:22:c9:42'>
+      <mac address='52:54:00:22:c9:42'/>
       <source address='192.168.0.1' port='5558'/>
     </interface>
     ...
     <interface type='client'>
-      <mac address='52:54:00:8b:c9:51'>
+      <mac address='52:54:00:8b:c9:51'/>
       <source address='192.168.0.1' port='5558'/>
     </interface>
   </devices>