[gtkmm-documentation] C++11: Use the override keyword.

[Murray Cumming <murrayc src gnome org>]

commit 8b872ec2b3fb75e693a14aab246e11266a83d049
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Oct 28 14:00:55 2015 +0100

    C++11: Use the override keyword.

 examples/others/exception/exceptiontest.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/others/exception/exceptiontest.cc b/examples/others/exception/exceptiontest.cc
index 69f4451..24ceaf1 100644
--- a/examples/others/exception/exceptiontest.cc
+++ b/examples/others/exception/exceptiontest.cc
@@ -19,7 +19,7 @@ public:
   explicit MyButton(const Glib::ustring& label);
 
 protected:
-  void on_clicked(); // signal handler
+  void on_clicked() override; // signal handler
 };
 
 class MyException1 {};