Change comment of percentage in BatteryState.msg

[RasmusLar]

For the BatteryState percentage to be correctly named, it should be in the range of 0-100. The current naming is misleading, as it suggests the percentage is in the range of 0-1, which is not a percentage, but a fraction.

Another option would be to change the name to ratio or charge_level and keep the value range to be 0-1, but that would be a breaking change.