PM / Documentation: suspend-and-cpuhotplug.txt: Fix typo

sysfs was expected in this context.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Acked-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>