[docs] [issue14129] Corrections for the "extending" doc

[Eli Bendersky]

Eli Bendersky <eliben at gmail.com> added the comment:

Thanks for the review. I'm going to do the commit now. Feel free to just fix it if any obvious mistakes remain.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue14129>
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