[gtkmm-documentation] Fix the listmodel example

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 49edd43ff32181bb70e66cf06ca0256ab7f9a807
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Tue Nov 13 14:52:39 2018 +0100

    Fix the listmodel example
    
    MyObject::create() must call Glib::make_refptr_for_instance().

 examples/book/listmodel/myobject.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/examples/book/listmodel/myobject.cc b/examples/book/listmodel/myobject.cc
index 3c28fd7..90f9922 100644
--- a/examples/book/listmodel/myobject.cc
+++ b/examples/book/listmodel/myobject.cc
@@ -26,7 +26,7 @@ m_property_label(*this, "label", label)
 
 Glib::RefPtr<MyObject> MyObject::create(int id, const Glib::ustring& label)
 {
-  return Glib::RefPtr<MyObject>(new MyObject(id, label));
+  return Glib::make_refptr_for_instance(new MyObject(id, label));
 }
 
 Glib::PropertyProxy<Glib::ustring> MyObject::property_label()