fix error in derivative term(dyp_dup) of spalding wall function

Author: dunapkt

SU2_CFD/src/solvers/CIncNSSolver.cpp

@@ -752,7 +752,7 @@ void CIncNSSolver::SetTau_Wall_WF(CGeometry *geometry, CSolver **solver_containe
 
         /* --- Gradient of function defined above wrt U_Tau --- */
 
-        const su2double dyp_dup = 1.0 + exp(-kappa * B) * (kappa * exp(kUp) - kappa - kUp - 0.5 * kUp * kUp);
+        const su2double dyp_dup = 1.0 + kappa * exp(-kappa * B) * (exp(kUp) - 1.0 - kUp - 0.5 * kUp * kUp);
         const su2double dup_dutau = - U_Plus / U_Tau;
         const su2double grad_diff = Density_Wall * WallDistMod / Lam_Visc_Wall - dyp_dup * dup_dutau;