ArduPlane: fix quadplane stopping speed equation

Author: eppravitra

ArduPlane/quadplane.cpp

@@ -2530,7 +2530,7 @@ void QuadPlane::vtol_position_controller(void)
         // calculate speed we should be at to reach the position2
         // target speed at the position2 distance threshold, assuming
         // Q_TRANS_DECEL is correct
-        const float stopping_speed = safe_sqrt(MAX(0, distance-position2_dist_threshold) * 2 * transition_decel) + position2_target_speed;
+        const float stopping_speed = safe_sqrt(MAX(0, distance-position2_dist_threshold) * 2 * transition_decel + sq(position2_target_speed));
 
         float target_speed = stopping_speed;