Important typo (changes semantics)

[[Imported from SVN: r4916]]

dune/solvers/iterationsteps/blockgsstep.cc

@@ -67,7 +67,7 @@ void BlockGSStep<OperatorType, DiscFuncType, BitVectorType>::iterate()
                     for (int k = 0; k < BlockSize; ++k)
 			matRes[j][k] = (k == j);
                 } else 
-                    matRes[j] == mat[i][i][j];
+                    matRes[j] = mat[i][i][j];
 
             }
             matRes.solve(v, r);