docs: re-fix stray /

Commit 6cb4acce reintroduced the bug fixed in commit d145fe3b.

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

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'>
         <seclabel relabel='no'/>
       </source>
       <target dev='hda' bus='ide'/>