mm: cleanup on the comments of zone_reclaim_stat

Signed-off-by: NWanpeng Li <liwp.linux@gmail.com>
Acked-by: NMinchan Kim <minchan@kernel.org>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

include/linux/mmzone.h

@@ -316,7 +316,7 @@ enum zone_type {
 struct zone_reclaim_stat {
 	/*
 	 * The pageout code in vmscan.c keeps track of how many of the
-	 * mem/swap backed and file backed pages are refeferenced.
+	 * mem/swap backed and file backed pages are referenced.
 	 * The higher the rotated/scanned ratio, the more valuable
 	 * that cache is.
 	 *