fix an error message. Patch by Uli Sack

[[Imported from SVN: r1632]]

dune/istl/diagonalmatrix.hh

@@ -692,7 +692,7 @@ namespace Dune {
     {
 #ifdef DUNE_FMatrix_WITH_CHECKING
       if (i!=row_)
-        DUNE_THROW(FMatrixError,"index is contained in pattern");
+        DUNE_THROW(FMatrixError,"index is not contained in pattern");
 #endif
       return *p_;
     }