[MaL/Fluid/Viscosity] Fix derivation.

MaterialLib/Fluid/Viscosity/TemperatureDependentViscosity.h

@@ -70,7 +70,7 @@ public:
     }
 
     /**
-     *  Get the partial differential of the viscosity with respect to
+     * Get the partial differential of the viscosity with respect to
      * temperature.
      * \param var_vals  Variable values  in an array. The order of its elements
      *                  is given in enum class PropertyVariableType.
@@ -81,7 +81,8 @@ public:
     {
         (void)var;
         const double T = var_vals[static_cast<int>(PropertyVariableType::T)];
-        return -_mu0 * std::exp(-(T - _temperature_c) / _temperature_v);
+        return -_mu0 / _temperature_v *
+               std::exp(-(T - _temperature_c) / _temperature_v);
     }
 
 private: