Docs/admin-guide/mm/damon/usage: update for changed initail_regions file input

A previous commit made init_regions debugfs file to use target index
instead of target id for specifying the target of the init regions.  This
commit updates the usage document to reflect the change.

Link: https://lkml.kernel.org/r/20211230100723.2238-3-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>