Fix the parameter list of object. _rpow_ (#GH-16477)

Author: hongweipeng

Doc/reference/datamodel.rst

@@ -2331,7 +2331,7 @@ left undefined.
             object.__rfloordiv__(self, other)
             object.__rmod__(self, other)
             object.__rdivmod__(self, other)
-            object.__rpow__(self, other)
+            object.__rpow__(self, other[, modulo])
             object.__rlshift__(self, other)
             object.__rrshift__(self, other)
             object.__rand__(self, other)