cmd_usb_mass_storage: Remove extra 'ums' string in the usage text

Currently the usage text for the 'ums' command looks like this:

Usage:
ums ums <USB_controller> [<devtype>] <devnum>  e.g. ums 0 mmc 0

,so remove the extra 'ums' in the text.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Marek Vasut <marex@denx.de>

common/cmd_usb_mass_storage.c

@@ -159,6 +159,6 @@ exit:
 
 U_BOOT_CMD(ums, 4, 1, do_usb_mass_storage,
 	"Use the UMS [User Mass Storage]",
-	"ums <USB_controller> [<devtype>] <devnum>  e.g. ums 0 mmc 0\n"
+	"<USB_controller> [<devtype>] <devnum>  e.g. ums 0 mmc 0\n"
 	"    devtype defaults to mmc"
 );