Fix wrong cpuio option name in documentation

Fix wrong cpuio option name in documentation

The name of an option for cpuio engine is "cpuchunks",
though the code internally uses "cpucycle" for variables.

(If the option name "cpuchunks" is the incorrect one,
I'll resubmit a patch that corrects the option name)

 # fio --enghelp=cpuio
 cpuload                 : Use this percentage of CPU
   cpuchunks             : Length of the CPU burn cycles (usecs)
   exit_on_io_done         : Exit when IO threads finish

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>