Readme correction and clarity change

[slepiavka]

1. Got rid of dBm for monitoring_status which should be dimensionless
2. Corrected the monitoring_status template, which was pointing to BLE rssi
3. Added some clarity about the sensor name. This is subjective but clarity usually helps.

[slepiavka]

I'd even argue that owlet_monitoring_status is redundant, since that state is already pushed by the main platform: owlet sensor.