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>

Documentation/cpu-hotplug.txt

@@ -285,7 +285,7 @@ A: This is what you would need in your kernel code to receive notifications.
 		return NOTIFY_OK;
 	}
 
-	static struct notifier_block foobar_cpu_notifer =
+	static struct notifier_block foobar_cpu_notifier =
 	{
 	   .notifier_call = foobar_cpu_callback,
 	};