[navierstokes][problem] Improve code comment

dumux/freeflow/navierstokes/problem.hh

@@ -482,7 +482,7 @@ public:
         GlobalPosition orientation(0.0);
         orientation[scv.dofAxis()] = 1.0;
 
-        // du/dy + dv/dx = alpha/sqrt(K) * (u_boundary-uPM)
+        // du/dy + dv/dx = beta * (u_boundary-uPM)
         // beta = alpha/sqrt(K)
         const Scalar betaBJ = asImp_().betaBJ(fvGeometry, scvf, orientation);
         const Scalar distanceNormalToBoundary = (scvf.ipGlobal() - scv.dofPosition()).two_norm();