docs: formatdomain: Close <source> on one of disk examples

Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/formatdomain.html.in

@@ -2881,6 +2881,7 @@
       <reservations managed='no'>
         <source type='unix' path='/path/to/qemu-pr-helper' mode='client'/>
       </reservations>
+    </source>
     <target dev='sda' bus='scsi'/>
     <address type='drive' controller='0' bus='0' target='3' unit='0'/>
   </disk>