[gtkmm-documentation] Add an override keyword

[Murray Cumming <murrayc src gnome org>]

commit 9ef39625cf3a386c345dc9e4db59d5069dfcdd4d
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Nov 26 12:24:18 2015 +0100

    Add an override keyword

 examples/others/treemodelcustom/exampletreemodel.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/others/treemodelcustom/exampletreemodel.h 
b/examples/others/treemodelcustom/exampletreemodel.h
index 45bc55f..2971565 100644
--- a/examples/others/treemodelcustom/exampletreemodel.h
+++ b/examples/others/treemodelcustom/exampletreemodel.h
@@ -43,7 +43,7 @@ protected:
    GType get_column_type_vfunc(int index) const override;
    void get_value_vfunc(const TreeModel::iterator& iter, int column, Glib::ValueBase& value) const override;
   
-   bool iter_next_vfunc(const iterator& iter, iterator& iter_next) const;
+   bool iter_next_vfunc(const iterator& iter, iterator& iter_next) const override;
 
    //TODO: Make sure that we make all of these const when we have made them all const in the TreeModel:
    bool iter_children_vfunc(const iterator& parent, iterator& iter) const override;