Documentation: cpu-hotplug: Fix sysfs mount instructions

The instructions for mounting sysfs are inconsistent in instructing to
create the directory '/sysfs' but then mounting sysfs to /sys.
Also, indentation is slightly off.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>