keep the lower case letter for flag info

Signed-off-by: yanggang <gang.yang@daocloud.io>

cmd/ctr/commands/commands_unix.go

@@ -32,11 +32,11 @@ func init() {
 		Usage: "disable use of pivot-root (linux only)",
 	}, cli.Int64Flag{
 		Name:  "cpu-quota",
-		Usage: "Limit CPU CFS quota",
+		Usage: "limit CPU CFS quota",
 		Value: -1,
 	}, cli.Uint64Flag{
 		Name:  "cpu-period",
-		Usage: "Limit CPU CFS period",
+		Usage: "limit CPU CFS period",
 	}, cli.StringFlag{
 		Name:  "rootfs-propagation",
 		Usage: "set the propagation of the container rootfs",