GTKMM Tutorial Slight Tweak

Today, I was going through the current stable Gtkmm tutorial, and found that one line is commented out, which should not be.

https://developer.gnome.org/gtkmm-tutorial/stable/sec-text-entry.html.en

Under section

In the examplewindow.h file, the line:

//bool on_completion_match(const Glib::ustring& key, const Gtk::TreeModel::const_iterator& iter);

Should not be commented out as this gives back an error when compiling. An easy mistake to find for anyone to fix, but should be addressed.

This is also apparent in the unstable tutorial as well FYI.

-- 
Steve Gruspier
Electrical Engineer