docs: Do not reference non-existent --dumb option

docs: Do not reference non-existent --dumb option

This appears to be a relic from GRUB legacy that used a --dumb option for
its terminal command. The proper way to do this in GRUB2 is to set the
terminal to "dumb" via the terminfo command.

Fixes: https://savannah.gnu.org/bugs/?66302
Reported-by: Jernej Jakob <jernej.jakob+savgnu@gmail.com>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>