Correct DESIGN to refer to 64 byte rolling window.

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>
Reviewed-by: Gabriel Filion <gabster@lelutin.ca>
Remove misleading -1 from BUP_WINDOWSIZE definition.

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>
Reviewed-by: Gabriel Filion <gabster@lelutin.ca>