Fix a doc bug

[SVN r15981]

doc/v2/reference_existing_object.html

@@ -143,7 +143,9 @@ Singleton& get_it()
 using namespace boost::python;
 BOOST_PYTHON_MODULE(singleton)
 {
-    def("get_it", get_it, reference_existing_object());
+    def("get_it", get_it,
+        return_value_policy<reference_existing_object>());
+
     class_<Singleton>("Singleton")
        .def("exchange", &Singleton::exchange)
        ;