Docs/admin-guide/mm/damon/usage: fix typos in references and commas

Fix typos including a unnecessary comma and incomplete ':ref:' keywords.

Link: https://lkml.kernel.org/r/20230616191742.87531-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>