[gtkmm-documentation] Remove unnecessary virtual keywords

[Murray Cumming <murrayc src gnome org>]

commit 4a3267963753a7e9cacce3912ef0e81913a40674
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Oct 28 14:03:38 2015 +0100

    Remove unnecessary virtual keywords

 examples/book/printing/advanced/previewdialog.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/book/printing/advanced/previewdialog.h b/examples/book/printing/advanced/previewdialog.h
index 0852f12..105cd18 100644
--- a/examples/book/printing/advanced/previewdialog.h
+++ b/examples/book/printing/advanced/previewdialog.h
@@ -36,7 +36,7 @@ protected:
 
   void on_hide() override;
 
-  virtual bool on_drawing_area_draw(const Cairo::RefPtr<Cairo::Context>& cr);
+  bool on_drawing_area_draw(const Cairo::RefPtr<Cairo::Context>& cr);
 
   //PrintOperationPreview signal handlers:
   void on_popreview_ready(const Glib::RefPtr<Gtk::PrintContext>& print_ctx);