mm/compaction: improve comment for compact_memory tunable knob handler

sysctl_compaction_handler() is the handler function for compact_memory
tunable knob under /proc/sys/vm, add the missing knob name to make this
more accurate in comment.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>