updated spec

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402747

dlib/matrix/matrix_abstract.h

@@ -165,7 +165,7 @@ namespace dlib
         ) const; 
         /*!
             ensures
-                - returns a copyable reference to the subexpression contained in *this.
+                - returns a reference to the expression contained in *this.
         !*/
 
     protected: