Fix example of calling parent methods

src/core/Class.leafdoc

@@ -71,7 +71,7 @@ You can call parent methods (including constructor) from corresponding child one
 ```
 var MyChildClass = MyClass.extend({
     initialize: function () {
-        MyClass.prototype.initialize.call("Yo");
+        MyClass.prototype.initialize.call(this, "Yo");
     },
 
     greet: function (name) {