HOWTO/manpage: update percentage explanation using '%'

82dbb8cb (HOWTO: add offset unit info for offset= option) added
how to specify offset= option in percentage format based on below post,
which was technically correct, but str_to_decimal() invoked via
option parser can convert '%' format to low-level representation.

This commit replaces 82dbb8cb using '%' syntax since this is
considered better for documentation. Note that size= option which
also supports percentage uses '%' in documentation.

in what units is offset option?
http://www.spinics.net/lists/fio/msg05969.html

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