docs: fix memory.low description in cgroup-v2.rst

The current cgroup-v2.rst file contains an incorrect description of when
memory is reclaimed from a cgroup that is using the 'memory.low'
mechanism. This fix simply corrects the text to reflect the actual
implementation.

Fixes: 7854207f

 ("mm/docs: describe memory.low refinements")
Signed-off-by: Jon Haslam <jonhaslam@fb.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>