doc: fix a typo about irq affinity

smp_affinity holds bitmask and smp_affinity_list holds list. So we
should write a list to smp_affinity_list, instead of smp_affinity.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>