KRain KRX8: use plug rather than power class for mains power

Power is an on/off indication. Plugged/unplugged makes more sense when the device is still on when running on battery.

PR #2948

custom_components/tuya_local/devices/krain_krx8_irrigation_controller.yaml

@@ -103,7 +103,7 @@ entities:
         name: sensor
         type: boolean
   - entity: binary_sensor
-    class: power
+    class: plug
     dps:
       - id: 131
         name: sensor