[docs] [issue37432] Fix a small param type in allocation.rst

[hai shi]

hai shi <shihai1991 at 126.com> added the comment:

Due to https://github.com/python/cpython/blob/master/Include/objimpl.h#L102,
`void PyObject_Del(PyObject *op)` should be `void PyObject_Del(void *op)`

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37432>
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