Docs/admin-guide/mm/damon/start: add --target_pid to DAMOS example command

The example command doesn't work [1] on the latest DAMON user-space tool,
since --damos_action option is updated to receive multiple arguments, and
hence cannot know if the final argument is for deductible monitoring
target or an argument for --damos_action option. Add --target_pid option
to let damo understand it is for target pid.

Link: https://lkml.kernel.org/r/20250916032339.115817-5-sj@kernel.org
Link: https://github.com/damonitor/damo/pull/32 [2]
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>