[docs] [issue22603] Fix a typo in the contextlib docs

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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keywords: +easy
nosy: +ncoghlan
stage:  -> commit review
type: crash -> enhancement

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22603>
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