admin-guide/cputopology.rst: Remove non-existed cpu-hotplug.txt

Since kernel commit ff58fa7f ("Documentation: Update CPU hotplug and move it to core-api"),
cpu_hotplug.txt has been removed. We should update it in here.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Link: https://lore.kernel.org/r/1624412269-13155-1-git-send-email-xuyang2018.jy@fujitsu.com


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