Fix comment typo for init_cma_reserved_pageblock

It seems the "it's" should be "its" here.
Signed-off-by: NLi Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

mm/page_alloc.c

@@ -768,7 +768,7 @@ void __init __free_pages_bootmem(struct page *page, unsigned int order)
 }
 
 #ifdef CONFIG_CMA
-/* Free whole pageblock and set it's migration type to MIGRATE_CMA. */
+/* Free whole pageblock and set its migration type to MIGRATE_CMA. */
 void __init init_cma_reserved_pageblock(struct page *page)
 {
 	unsigned i = pageblock_nr_pages;