Device.appendOrUpdate RSSI filter typo

When a new device is added via Device.appendOrUpdate, the RSSI check compares the `newDevice` against itself in the first logical expression ([line 34](https://github.com/adafruit/Adafruit_BLESniffer_Python/blob/master/SnifferAPI/Devices.py#L34)). I believe it should compare the new device against the existing one (like it does in the second logical expression).

It looks like the new device is to be added if its RSSI is less than or greater than the existing device's (by some threshold). If so, wouldn't switching the two devices in the second logical expression make for a better filter? For example,
```
if (newDevice.RSSI < (existingDevice.RSSI - 5)) or (newDevice.RSSI > (existingDevice.RSSI+2)):
```

Start in SnifferAPI/Devices.py at line 34 and inspect Device.appendOrUpdate, focusing on how the RSSI comparison uses newDevice and existingDevice. Confirm the intended threshold behavior from the surrounding logic, then verify that the filter compares the new device with the existing device in both expressions and preserves the correct add/update behavior.

Written by the indexing model from the issue text.