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

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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assignee: docs at python -> terry.reedy
nosy: +terry.reedy
type: enhancement -> behavior
versions:  -Python 3.2, Python 3.3

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