[issue31995] Set operations documentation error

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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nosy: +rhettinger, serhiy.storchaka
versions:  -Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31995>
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