[gtkmm-documentation] Flowbox example: Avoid a compiler error.

[Murray Cumming <murrayc src gnome org>]

commit 3185c2efa5ab6d55175a2aec5f06c7df4a1848e1
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Apr 9 09:18:08 2014 +0200

    Flowbox example: Avoid a compiler error.

 examples/book/flowbox/examplewindow.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/book/flowbox/examplewindow.cc b/examples/book/flowbox/examplewindow.cc
index 1debbef..523b2f6 100644
--- a/examples/book/flowbox/examplewindow.cc
+++ b/examples/book/flowbox/examplewindow.cc
@@ -33,7 +33,7 @@ ExampleWindow::ExampleWindow()
 
   // Color swatches
   fill_color_names();
-  for (int i(0); i != m_color_names.size(); ++i)
+  for(std::size_t i = 0; i != m_color_names.size(); ++i)
   {
     Gtk::Button* color_swatch = create_color_swatch(i);
     m_flow_box.add(*color_swatch);