[Cleanup] Fix a comment

src/sand-wedge.cc

@@ -404,7 +404,7 @@ int main(int argc, char *argv[]) {
     Vector a_initial(leafVertexCount);
     a_initial = 0.0;
     {
-      // We solve Ma = ell0 - [Au + Cv + Psi(v)]
+      // We solve Ma = ell0 - [Au + Cv + DPsi(v)]
       Vector accelerationRHS(leafVertexCount);
       {
         accelerationRHS = 0.0;