cmd: ufs: Correct the help text

Remove the additional space and use "sub-system" for consistency
with other commands like "scsi" and "usb".

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>