docs: virtiofs: add missing aposthrophe

Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

docs/formatdomain.html.in

@@ -3981,7 +3981,7 @@
     <readonly/>
   </filesystem>
   <filesystem type='mount' accessmode='passthrough'>
-      <driver type='virtiofs queue='1024'/>
+      <driver type='virtiofs' queue='1024'/>
       <binary path='/usr/libexec/virtiofsd' xattr='on'>
          <cache mode='always'/>
          <lock posix='on' flock='on'/>