Fix Issue13147 - Correct the Multiprocessing Pool.map_async method signature.

Remove the error_callback which is not present in 2.7. Patch by Jyrki
Pulliainen.

Doc/library/multiprocessing.rst

@@ -1602,7 +1602,7 @@ with the :class:`Pool` class.
       the process pool as separate tasks.  The (approximate) size of these
       chunks can be specified by setting *chunksize* to a positive integer.
 
-   .. method:: map_async(func, iterable[, chunksize[, callback[, error_callback]]])
+   .. method:: map_async(func, iterable[, chunksize[, callback]])
 
       A variant of the :meth:`.map` method which returns a result object.