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



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.
1 file changed