doc: fix incorrect option in send-key

* tools/virsh.pod: fix a option typo of send-key section.
Signed-off-by: NAlex Jia <ajia@redhat.com>

tools/virsh.pod

@@ -356,7 +356,7 @@ The numeric values are those defined by the OS-X keyboard input
 subsystem. The symbolic names match the corresponding OS-X key
 constant macro names
 
-=item B<xtkbd>
+=item B<xt_kbd>
 
 The numeric values are those defined by the Linux KBD device.
 These are a variant on the original XT codeset, but often with