PH-W218: temperature appears to be in Celsius

No feedback received on #1997, but the reading seems to be in the
Celsius range, as it would be frozen if Fahrenheit.

custom_components/tuya_local/devices/phw218_waterquality_monitor.yaml

@@ -13,6 +13,7 @@ primary_entity:
       class: measurement
       mapping:
         - scale: 10
+      unit: C
 secondary_entities:
   - entity: sensor
     name: pH