virsh: add missing help info of --source to domifaddr

commit b4b5c82c forgot to add this.
Signed-off-by: NChen Hanxiao <chenhanxiao@gmail.com>

tools/virsh-domain-monitor.c

@@ -2160,7 +2160,7 @@ static const vshCmdOptDef opts_domifaddr[] = {
     {.name = "source",
      .type = VSH_OT_STRING,
      .flags = VSH_OFLAG_NONE,
-     .help = N_("address source: 'lease' or 'agent'")},
+     .help = N_("address source: 'lease', 'agent', or 'arp'")},
     {.name = NULL}
 };