mm/vmscan: cleanup comment error in balance_pgdat

Signed-off-by: Wanpeng Li <liwp.linux@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

mm/vmscan.c

@@ -2864,7 +2864,7 @@ loop_again:
 				 * consider it to be no longer congested. It's
 				 * possible there are dirty pages backed by
 				 * congested BDIs but as pressure is relieved,
-				 * spectulatively avoid congestion waits
+				 * speculatively avoid congestion waits
 				 */
 				zone_clear_flag(zone, ZONE_CONGESTED);
 				if (i <= *classzone_idx)