[docs] [issue31304] Update doc for starmap_async error_back kwarg

[tamas]

New submission from tamas:

The documentation suggests that multiprocessing.pool.Pool. starmap_async has a kwarg called 'error_back', but it actually has a kwarg called 'error_callback' to handle errors, just like map_async.

Link:
https://docs.python.org/3.7/library/multiprocessing.html#multiprocessing.pool.Pool.starmap_async

----------
assignee: docs at python
components: Documentation
messages: 300999
nosy: docs at python, tamas
priority: normal
severity: normal
status: open
title: Update doc for starmap_async error_back kwarg
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31304>
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