mm: fix typo of MIGRATE_RESERVE in comment

Found it when I want to jump to the definition of MIGRATE_RESERVE ctags.
Signed-off-by: NBaoquan He <bhe@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -426,7 +426,7 @@ struct zone {
 	const char		*name;
 
 	/*
-	 * Number of MIGRATE_RESEVE page block. To maintain for just
+	 * Number of MIGRATE_RESERVE page block. To maintain for just
 	 * optimization. Protected by zone->lock.
 	 */
 	int			nr_migrate_reserve_block;