Trivial: Fix typo in Py_DECREF comment

[jkrejcha]

Py_DECREF's comment incorrectly references _Py_IncRef which isn't used by decref. Both functions don't accept NULL but only _Py_DecRef is relevant for the decref function

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[StanFromIreland]

Why change a comment that is wrong to one that is also wrong?

This is explained in a previous comment in the file:
If it may be NULL, use Py_XINCREF/Py_XDECREF instead.

I think this comment should be completely changed or removed.

[picnixz]

The comment is more for internal purposes and implementation details explaining why we are using _Py_DecRef instead of Py_DecRef and for compatibility reasons.

The caller is not meant to pass NULL when using Py_DECREF at all, but the implementation may appear to allow it (at least until Python 3.10).

[jkrejcha]

Indeed this appears to me to be the case. Py_DecRef (notably not Py_DECREF) is just be a thin wrapper around Py_XDECREF.