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

[Eli Bendersky]

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

"The new method calls the tp_alloc slot to allocate memory"

tp_alloc needs formatting here, similarly to the way it's done in other places

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