mm/page_alloc.c: fix typos in comments

Link: http://lkml.kernel.org/r/1515485774-4768-1-git-send-email-zhangshile@gmail.com


Signed-off-by: Shile Zhang <zhangshile@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>