Simple Documentation fix: Missing link to return type class. (GH-21291)

Just a simple documentation fix: apply_async and map_async return a "multiprocessing.pool.AsyncResult Object", not a "result object".
(cherry picked from commit f9bf0157)

Co-authored-by: Volker-Weissmann <39418860+Volker-Weissmann@users.noreply.github.com>