tools/vm/page_owner: use page_owner_sort in the use example

The example in comment does not useable because the output binary is
named "page_owner_sort", not "sort".

Also add a reference to Documentation/vm/page_owner.rst

Link: http://lkml.kernel.org/r/1546515361-8317-1-git-send-email-miles.chen@mediatek.com


Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>