Fixed typo in spec

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402334

dlib/gui_widgets/fonts_abstract.h

@@ -479,7 +479,7 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    font* get_native_font (
+    const font* get_native_font (
     );
     /*!
         ensures