Fixed typo in spec

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403585

dlib/matrix/matrix_utilities_abstract.h

@@ -716,7 +716,7 @@ namespace dlib
     );
     /*!
         requires
-            - a.nr() == b.nr()
+            - a.nc() == b.nc()
             - a and b both contain the same type of element
         ensures
             - This function joins two matrices together by concatenating their columns.