Documentation/cpu-hotplug.txt: fix a typo in example code

As the notifier_block name (i.e.  foobar_cpu_notifer) is different from
the parameter (i.e.foobar_cpu_notifier) of register function, that is
definitely error and it also makes readers confused.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>