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: NHu Tao <hutao@cn.fujitsu.com>
Acked-by: NRob Landley <rob@landley.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Documentation/IRQ-affinity.txt

@@ -57,8 +57,8 @@ i.e counters for the CPU0-3 did not change.
 
 Here is an example of limiting that same irq (44) to cpus 1024 to 1031:
 
-[root@moon 44]# echo 1024-1031 > smp_affinity
-[root@moon 44]# cat smp_affinity
+[root@moon 44]# echo 1024-1031 > smp_affinity_list
+[root@moon 44]# cat smp_affinity_list
 1024-1031
 
 Note that to do this with a bitmask would require 32 bitmasks of zero