[bugfix] correct indices for symmetrization of matrix quadraticIntegrals

dune/mmdg/dg.hh

@@ -174,7 +174,7 @@ public:
             }
 */
             //NOTE: probably unnecessary
-            quadraticIntregrals[i][j] = quadraticIntregrals[j][i];
+            quadraticIntregrals[j][i] = quadraticIntregrals[i][j];
           }
         }
         std::cout << "linearIntegrals:\n" << linearIntegrals << "\n\n";