Documentation: fix typo for core-api/cpu_hotplug documentation

fix typo for core-api/cpu_hotplug documentation

Signed-off-by: Yaohui Wang <dillionmango@gmail.com>
Link: https://lore.kernel.org/r/20200722073818.20605-1-dillionmango@gmail.com


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