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

[Roundup Robot]

Roundup Robot added the comment:

New changeset 6575074aae1c by Serhiy Storchaka in branch '3.3':
Issue #18757: Improved cross-references in the concurrent package.
http://hg.python.org/cpython/rev/6575074aae1c

New changeset 383d85249997 by Serhiy Storchaka in branch 'default':
Issue #18757: Improved cross-references in the concurrent package.
http://hg.python.org/cpython/rev/383d85249997

New changeset c24941251473 by Serhiy Storchaka in branch '2.7':
Issue #18757: Improved cross-references in the concurrent package.
http://hg.python.org/cpython/rev/c24941251473

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nosy: +python-dev

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<http://bugs.python.org/issue18757>
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