HOWTO: fix wrong kb_base= description

Definition of [KMGTP] and [KMGTP]i for kb_base=1000 are opposite.
It instead describes how it works when kb_base=1024 which is default.

This was originally spotted by Mikhail Terekhov in the mailing list
(in a related thread about a week ago, and the patch hasn't been sent).
http://www.spinics.net/lists/fio/msg06151.html

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>