Fix sample TPM XML

Fix an error in the sample TPM XML.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>

docs/formatdomain.html.in

@@ -4735,7 +4735,7 @@ qemu-kvm -net nic,model=? /dev/null
   <devices>
     <tpm model='tpm-tis'>
       <backend type='passthrough'>
-        <backend path='/dev/tpm0'/>
+        <device path='/dev/tpm0'/>
       </backend>
     </tpm>
   </devices>