Correct mistake in mooneyrivlindensity

Correct mistake so the formula is the one from the paper here: https://www.sciencedirect.com/science/article/abs/pii/S0924424715002022

dune/elasticity/materials/mooneyrivlindensity.hh

@@ -130,7 +130,7 @@ public:
       {
         //mooneyrivlin_energy is "square"
         field_type detFMinus1 = detF - 1;
-        return strainEnergy + mooneyrivlin_k* detFMinus1 * detFMinus1;
+        return strainEnergy + 0.5 * mooneyrivlin_k* detFMinus1 * detFMinus1;
       }
     }
   }