Fixed typo in error message

dlib/matrix/matrix.h

@@ -436,7 +436,7 @@ namespace dlib
             DLIB_ASSERT(lhs.nc() == rhs.nc() &&
                    lhs.nr() == rhs.nr(), 
                 "\tconst matrix_exp operator-(const matrix_exp& lhs, const matrix_exp& rhs)"
-                << "\n\tYou are trying to add two incompatible matrices together"
+                << "\n\tYou are trying to subtract two incompatible matrices"
                 << "\n\tlhs.nr(): " << lhs.nr()
                 << "\n\tlhs.nc(): " << lhs.nc()
                 << "\n\trhs.nr(): " << rhs.nr()