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

(cherry picked from commit abc0c4f)

Co-authored-by: HongWeipeng <961365124@qq.com>

Author: miss-islington

Doc/reference/datamodel.rst

@@ -2318,7 +2318,7 @@ 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)