Keep pid call uniform with update function

Author: iysheng

examples/water_boiler/water_boiler.py

@@ -43,7 +43,7 @@ def update(self, boiler_power, dt):
         current_time = time.time()
         dt = current_time - last_time
 
-        power = pid(water_temp)
+        power = pid(water_temp, dt)
         water_temp = boiler.update(power, dt)
 
         x += [current_time - start_time]