mmzone: fix comment typo coelesce -> coalesce

Signed-off-by: NWang YanQing <udknight@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

include/linux/mmzone.h

@@ -30,7 +30,7 @@
 /*
  * PAGE_ALLOC_COSTLY_ORDER is the order at which allocations are deemed
  * costly to service.  That is between allocation orders which should
- * coelesce naturally under reasonable reclaim pressure and those which
+ * coalesce naturally under reasonable reclaim pressure and those which
  * will not.
  */
 #define PAGE_ALLOC_COSTLY_ORDER 3