virsh: Fix on_shutdown to on_poweroff

The 'on_shutdown' was the name in Xen, there was never such option in
libvirt's domain XML.
Reported-by: NRuben Kerkhof <ruben@rubenkerkhof.com>
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

tools/virsh.pod

@@ -2398,7 +2398,7 @@ succeed, and may take a variable length of time depending on what
 services must be shutdown in the domain.
 
 The exact behavior of a domain when it shuts down is set by the
-I<on_shutdown> parameter in the domain's XML definition.
+I<on_poweroff> parameter in the domain's XML definition.
 
 If I<domain> is transient, then the metadata of any snapshots will
 be lost once the guest stops running, but the snapshot contents still