cmd: pxe: Update the command help

Currently the "help" displays pxe command help text like this:

  => help
  ...
  printenv  - print environment variables
  pxe       - commands to get and boot from pxe files
  To use IPv6 add -ipv6 parameter
  qfw       - QEMU firmware interface
  ...

This does not read clearly. Remove the IPv6 stuff as it is in
the detailed help text so that it fits just a single line.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>