[issue17586] fix typo in contextlib.rst

[Roundup Robot]

New submission from Roundup Robot:

New changeset 18e699c4d8c0 by Ned Deily in branch 'default':
Issue #17586: fix typo in contextlib.rst
http://hg.python.org/cpython/rev/18e699c4d8c0

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nosy: +python-dev

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17586>
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