[3.8] Fix typo in multiprocessing.pool.AsyncResult.successful doc. (GH-17932) (GH...

[3.8] Fix typo in multiprocessing.pool.AsyncResult.successful doc. (GH-17932) (GH-18015)

Since 3.7 `successful` raises a `ValueError` as explained in the next text block from the documentation:

_Changed in version 3.7: If the result is not ready, ValueError is raised instead of AssertionError._

No issue associated with this PR.
Should be backported in 3.7 and 3.8.
(cherry picked from commit dc0284ee8f7a270b6005467f26d8e5773d76e959)

Co-authored-by: Antoine <43954001+awecx@users.noreply.github.com>
Automerge-Triggered-By: @pitrou