[docs] [issue18757] Fix internal references for concurrent modules

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Thank you for the review Ezio.

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resolution:  -> fixed
stage: commit review -> committed/rejected
status: open -> closed

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18757>
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