Fix typo in documentation

As reported by Dennis van Raaij in
 #48

Author: setton

docs/gtkada_ug/intro.rst

@@ -35,7 +35,7 @@ with your OpenGL library, for more information.
 The following Internet sites will always contain the latest public packages for
 `GtkAda`, `gtk+`, `Glade` and `Cairo`
 
-* `https://github.com/AdaCore/gtkada <https://github.com/AdaCore/gtkada`_
+* `https://github.com/AdaCore/gtkada <https://github.com/AdaCore/gtkada>`_
 
 * `http://www.gtk.org/ <http://www.gtk.org/>`_