Fix unit_of_measurement of illuminance

[martinhpedersen]

I noticed this warning in my logs:

Entity sensor.fh_17_illuminance (<class 'homeassistant.components.mqtt.sensor.MqttSensor'>) is using native unit of measurement 'Lux' which is not a valid unit for the device class ('illuminance') it is using; expected one of ['lx']; Please update your configuration if your entity is manually configured, otherwise create a bug report at https://github.com/home-assistant/core/issues?q=is%3Aopen+is%3Aissue+label%3A%22integration%3A+mqtt%22

Not sure if it has any practical implications other than silencing the warning, but I guess it's worth fixing anyway 🙂