correct air index unit (#12730)

AirVisual's air index unit is AQI (Air Quality Index), not PSI (Pressure per Square Inch). More details can be found at https://airvisual.com/

Cheers,

homeassistant/components/sensor/airvisual.py

@@ -210,7 +210,7 @@ class AirQualityIndexSensor(AirVisualBaseSensor):
     @property
     def unit_of_measurement(self):
         """Return the unit the value is expressed in."""
-        return 'PSI'
+        return 'AQI'
 
     def update(self):
         """Update the status of the sensor."""