Documentation: core-api/cpu_hotplug: fix a typo

fourV CPUs should be four CPUs.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Link: https://lore.kernel.org/r/20200904101902.29560-1-song.bao.hua@hisilicon.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>