docs: remove stray /

Commit e5a84d74 added a new attribute in the wrong location;
commit c8b9fa74 fixed the missing / at the end but not the extra
/ in the middle.

* docs/formatdomain.html.in (elementsDisks): Fix another typo.

docs/formatdomain.html.in

@@ -947,7 +947,7 @@
   <devices>
     <disk type='file' snapshot='external'>
       <driver name="tap" type="aio" cache="default"/>
-      <source file='/var/lib/xen/images/fv0'/ startupPolicy='optional'/>
+      <source file='/var/lib/xen/images/fv0' startupPolicy='optional'/>
       <target dev='hda' bus='ide'/>
       <iotune>
         <total_bytes_sec>10000000</total_bytes_sec>