src/sage/rings/number_field/number_field_element.pyx: Update doctest output (trac->github)

Author: Matthias Koeppe

src/sage/rings/number_field/number_field_element.pyx

@@ -111,7 +111,7 @@ def is_NumberFieldElement(x):
         doctest:warning...
         DeprecationWarning: is_NumberFieldElement is deprecated;
         use isinstance(..., sage.structure.element.NumberFieldElement) instead
-        See https://trac.sagemath.org/34931 for details.
+        See https://github.com/sagemath/sage/issues/34931 for details.
         False
         sage: k.<a> = NumberField(x^7 + 17*x + 1)
         sage: is_NumberFieldElement(a+1)