irq/Documentation: Correct result of echnoing 5 to smp_affinity

This command:

  echo 5 > /proc/irq/10/smp_affinity

means only the first and third (not fourth) CPUs can handle irqs
That is, CPU0 is the first CPU and CPU2 is the third cpu

Signed-off-by: John Kacur <jkacur@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1466168715-8410-1-git-send-email-jkacur@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>