[gtkmm] incorrect link in online docs

[Trevor Spiteri <tspiteri ieee org>]

I was browsing the online documentation for gtkmm and found an incorrect
link in the page
http://gtkmm.sourceforge.net/gtkmm2/docs/reference/html/classGtk_1_1Window.html

The constructor for Gtk::Window takes an optional WindowType argument.
Clicking on WindowType takes me to Gdk::WindowType instead of
Gtk::WindowType.

Thanks

Trevor Spiteri