docs/mm: concepts.rst: Correct the threshold to low watermark

It should be "low watermark" where we wake up kswapd daemon.

Signed-off-by: Liao Pingfang <winndows@163.com>
Link: https://lore.kernel.org/r/1609053319-3112-1-git-send-email-winndows@163.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>