Rover: fixing a bug the Rover simulation

Just a small change to ensure the max_wheel_turn parameter is used
instead of a hard coded value.

libraries/SITL/SIM_Rover.cpp

@@ -54,7 +54,7 @@ float SimRover::turn_circle(float steering)
     if (fabsf(steering) < 1.0e-6) {
         return 0;
     }
-    return turning_circle * sinf(radians(35)) / sinf(radians(steering*35));
+    return turning_circle * sinf(radians(max_wheel_turn)) / sinf(radians(steering*max_wheel_turn));
 }
 
 /*