bpo-31304: Update starmap_async documentation. (GH-4168)

Update the kwarg in the documentation of `multiprocessing.pool.Pool.starmap_async`, from `error_back` to `error_callback` to match the source code..

Doc/library/multiprocessing.rst

@@ -2181,7 +2181,7 @@ with the :class:`Pool` class.
 
       .. versionadded:: 3.3
 
-   .. method:: starmap_async(func, iterable[, chunksize[, callback[, error_back]]])
+   .. method:: starmap_async(func, iterable[, chunksize[, callback[, error_callback]]])
 
       A combination of :meth:`starmap` and :meth:`map_async` that iterates over
       *iterable* of iterables and calls *func* with the iterables unpacked.