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>