docs: Add missing / to closing tag

The iothread example for virtio-scsi should be
<driver iothread='4'/> rather than <driver iothread='4'>
for the XML to be valid.

docs/formatdomain.html.in

@@ -3166,7 +3166,7 @@
       <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
     </controller>
     <controller type='scsi' index='0' model='virtio-scsi'>
-      <driver iothread='4'>
+      <driver iothread='4'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
     </controller>
     ...