Mustool MT15/MT29: fix scaling

- conversion from mg to µg is the opposite direction

Issue #3209

custom_components/tuya_local/devices/mustool_mt15mt29_airbox.yaml

@@ -59,7 +59,7 @@ entities:
         unit: µg/m³
         class: measurement
         mapping:
-          - scale: 1000
+          - scale: 0.001
   - entity: sensor
     class: pm25
     dps:
@@ -112,7 +112,7 @@ entities:
         unit: µg/m³
         class: measurement
         mapping:
-          - scale: 1000
+          - scale: 0.001
   - entity: sensor
     class: carbon_monoxide
     category: diagnostic