pythongh-111933: fix broken link to A.Neumaier article (pythongh-111937)

Author: skirpichev

Python/bltinmodule.c

@@ -2611,7 +2611,10 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
             }
             if (PyFloat_CheckExact(item)) {
                 // Improved Kahan–Babuška algorithm by Arnold Neumaier
-                // https://www.mat.univie.ac.at/~neum/scan/01.pdf
+                // Neumaier, A. (1974), Rundungsfehleranalyse einiger Verfahren
+                // zur Summation endlicher Summen.  Z. angew. Math. Mech.,
+                // 54: 39-51. https://doi.org/10.1002/zamm.19740540106
+                // https://en.wikipedia.org/wiki/Kahan_summation_algorithm#Further_enhancements
                 double x = PyFloat_AS_DOUBLE(item);
                 double t = f_result + x;
                 if (fabs(f_result) >= fabs(x)) {