Change battery voltage to be in volts or mV

[bharvey88]

warning in home assistant logs:

Entity sensor.apollo_plt_1_99284c_soil_moisture (<class 'homeassistant.components.esphome.sensor.EsphomeSensor'>) is using native unit of measurement '%' which is not a valid unit for the device class ('voltage') it is using; expected one of ['V', 'mV']; 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+esphome%22

suggestion:

to change the yaml for this sensor to use volts or mV