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

[Tshepang Lekhonkhobe]

New submission from Tshepang Lekhonkhobe:

Also, sentence starting with "Due to the" does not flow so nice for me... maybe a comma is needed after "CPython"?

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type:  -> enhancement

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16978>
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