Fix Morgan SCARA S2 calculation

Marlin/Marlin_main.cpp

     else
       C2 = (HYPOT2(sx, sy) - (L1_2 + L2_2)) / (2.0 * L1 * L2);
 
-    S2 = sqrt(sq(C2) - 1);
+    S2 = sqrt(1 - sq(C2));
 
     // Unrotated Arm1 plus rotated Arm2 gives the distance from Center to End
     SK1 = L1 + L2 * C2;