[3.7] Fix typo in multiprocessing.pool.AsyncResult.successful doc. (GH-17932) (GH-18016)

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 dc0284e)


Co-authored-by: Antoine <[email protected]>

Automerge-Triggered-By: @pitrou

Author: miss-islington

Doc/library/multiprocessing.rst

@@ -2252,7 +2252,7 @@ with the :class:`Pool` class.
    .. method:: successful()
 
       Return whether the call completed without raising an exception.  Will
-      raise :exc:`AssertionError` if the result is not ready.
+      raise :exc:`ValueError` if the result is not ready.
 
 The following example demonstrates the use of a pool::