refactor(README): use unregistered_devices instead of devices

[DemidenkoAndrey]

unregistered_devices returns hash, but devices returns array of tokens Change token key from string key to symbol

[Dellybro]

Whats the different between the array of unregistered devices and the normal array of devices

[DemidenkoAndrey]

@Dellybro unregistered_devices returns array of hash - { token: token, timestamp: timestamp }, and in example we have hash, but devices method returns array of tokens(array of strings)