Minor typo

docs/advanced/functions.rst

@@ -438,7 +438,7 @@ To explicitly enable or disable this behaviour, using the
     py::class_<Cat>(m, "Cat").def(py::init<>());
     m.def("bark", [](Dog *dog) -> std::string {
         if (dog) return "woof!"; /* Called with a Dog instance */
-        else return "(no dog)"; /* Called with None, d == nullptr */
+        else return "(no dog)"; /* Called with None, dog == nullptr */
     }, py::arg("dog").none(true));
     m.def("meow", [](Cat *cat) -> std::string {
         // Can't be called with None argument