pid: fix derivative calculation

Reviewed-on: M-Labs/thermostat#40
Co-Authored-By: topquark12 <aw@m-labs.hk>
Co-Committed-By: topquark12 <aw@m-labs.hk>

src/pid.rs

@@ -85,7 +85,7 @@ impl Controller {
             None =>
                 0.0,
             Some(last_input) =>
-                f64::from(self.parameters.kd) * (input - last_input) / time_delta,
+                f64::from(self.parameters.kd) * (last_input - input) / time_delta,
         };
         self.last_input = Some(input);