kcsan: Use URL link for pointing access-marking.txt

For consistency within kcsan.rst, use a URL link as the same as in
section "Data Races".

Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>