Docs/admin-guide/mm/damon/usage: fix the example code snip

The workflow example code is not working since it got the file names
wrong. So fix this.

Fixes: b1840272

 ("Docs/admin-guide/mm/damon/usage: document DAMON sysfs interface")
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
Link: https://lore.kernel.org/r/20220823114053.53305-1-ryncsn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>