[gtkmm-documentation] Small const improvement.

[Murray Cumming <murrayc src gnome org>]

commit 4b146548c3293a342c3991769047bfb7efa05753
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon May 16 20:19:01 2016 +0200

    Small const improvement.

 examples/others/calendar/calendar.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/others/calendar/calendar.cc b/examples/others/calendar/calendar.cc
index 318b97d..9618c0a 100644
--- a/examples/others/calendar/calendar.cc
+++ b/examples/others/calendar/calendar.cc
@@ -119,7 +119,7 @@ void CalendarExample::toggle_flag(Gtk::CheckButton *toggle)
 void CalendarExample::on_font_button_font_set()
 {
   const auto font_name = font_button_->get_font_name();
-  Pango::FontDescription font_desc(font_name);
+  const Pango::FontDescription font_desc(font_name);
   const auto font_family = font_desc.get_family();
   const auto font_size = font_desc.get_size();
   const auto css =