[docs] [issue16978] fix grammar in 'threading' documentation

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8886d7ca159b by Ezio Melotti in branch '2.7':
#16978: rephrase sentence and fix typo.  Initial patch by Tshepang Lekhonkhobe.
http://hg.python.org/cpython/rev/8886d7ca159b

New changeset 4a1a88d25fec by Ezio Melotti in branch '3.2':
#16978: rephrase sentence and fix typo.  Initial patch by Tshepang Lekhonkhobe.
http://hg.python.org/cpython/rev/4a1a88d25fec

New changeset ad0af795c345 by Ezio Melotti in branch '3.3':
#16978: merge with 3.2.
http://hg.python.org/cpython/rev/ad0af795c345

New changeset e1118ff088be by Ezio Melotti in branch 'default':
#16978: merge with 3.3.
http://hg.python.org/cpython/rev/e1118ff088be

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nosy: +python-dev

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