Btrfs-progs: fix wrong usage of btrfs subvolume list command

Since the uuid output function has been implemented, we should update
the usage to tell the users.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>

cmds-subvolume.c

@@ -258,10 +258,11 @@ static int cmd_subvol_delete(int argc, char **argv)
 }
 
 static const char * const cmd_subvol_list_usage[] = {
-	"btrfs subvolume list [-ps] <path>",
+	"btrfs subvolume list [-pu] [-s 0|1] <path>",
 	"List subvolumes (and snapshots)",
 	"",
 	"-p           print parent ID",
+	"-u           print the uuid of subvolumes (and snapshots)",
 	"-s value     list snapshots with generation in ascending/descending order",
 	"             (1: ascending, 0: descending)",
 	NULL