Fixed clobbered output of the "help usb" command

The "usb help" doesn't format the output correctly:

=> help usb
usb - USB sub-system

Usage:
usb reset - reset (rescan) USB controller
usb stop [f]  - stop USB [f]=force stop
usb tree  - show USB device tree
usb info [dev] - show available USB devices
usb storage  - show details of USB storage devices
usb dev [dev] - show or set current USB storage device
usb part [dev] - print partition table of one or all USB storage devices
usb read addr blk# cnt - read `cnt' blocks starting at block `blk#'
    to memory address `addr'usb write addr blk# cnt - write `cnt'
blocks starting at block `blk#' from memory address `addr'
=>

With fix below applied, the output is correct:

=> help usb
usb - USB sub-system

Usage:
usb reset - reset (rescan) USB controller
usb stop [f]  - stop USB [f]=force stop
usb tree  - show USB device tree
usb info [dev] - show available USB devices
usb storage  - show details of USB storage devices
usb dev [dev] - show or set current USB storage device
usb part [dev] - print partition table of one or all USB storage devices
usb read addr blk# cnt - read `cnt' blocks starting at block `blk#'
    to memory address `addr'
usb write addr blk# cnt - write `cnt' blocks starting at block `blk#'
    from memory address `addr'
=>

Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>