AP_Motors: minor format fix to tailsitter

libraries/AP_Motors/AP_MotorsTailsitter.cpp

@@ -161,8 +161,8 @@ void AP_MotorsTailsitter::output_armed_stabilizing()
     }
 
     // calculate left and right throttle outputs
-    _thrust_left  = throttle_thrust + roll_thrust*0.5;
-    _thrust_right = throttle_thrust - roll_thrust*0.5;
+    _thrust_left  = throttle_thrust + roll_thrust*0.5f;
+    _thrust_right = throttle_thrust - roll_thrust*0.5f;
 
     // if max thrust is more than one reduce average throttle
     thrust_max = MAX(_thrust_right,_thrust_left);