von mises stress formula is incorrect

Describe the bug
SU2 gets the equivalent stress in plane stress problem using the following formula:

template<class T>
static su2double VonMisesStress(unsigned short nDim, const T& stress) {
if (nDim == 2) {
  su2double Sxx = stress[0], Syy = stress[1], Sxy = stress[2];

  su2double S1, S2; S1 = S2 = (Sxx+Syy)/2;
  su2double tauMax = sqrt(pow((Sxx-Syy)/2, 2) + pow(Sxy,2));
  S1 += tauMax;
  S2 -= tauMax;

  return sqrt(S1*S1+S2*S2-2*S1*S2);
}

But I think there is a mistake.According to the formula of Von Mises stress：
https://ansyshelp.ansys.com/public/a...iv_Stress.html
The code should be ：

return sqrt(S1*S1+S2*S2-S1*S2);

Please let me know if my understanding is incorrect. Thank you.