examples: Improve scanner_advanced.py (#47)

- `BLEDevice.rssi` is deprecated since version 0.19.0 (see https://bleak.readthedocs.io/en/latest/api/index.html#bleak.backends.device.BLEDevice.rssi) switched to using the advertisement rssi (see https://bleak.readthedocs.io/en/latest/api/scanner.html#bleak.BleakScanner.discover).
- Battery has a "&" symbol after it instead of a percentage symbol (typo?).