mac addresses appear to be case sensitive (trivial)

[eichin]

After cut&pasting my BLE device address from gattool where it's all upper case, I got

[A4:C1:38:CE:0D:F5] Connection failed: Device does not exist, check adapter name and MAC address.

After running ./gattctl.py --discover I noticed the output was all lower case, so on a hunch, I downcased it and got

[a4:c1:38:ce:0d:f5] Connected

(yay!) I just suggest calling it out in the example, or throwing in a .lower() and avoiding the issue, since other (linux, at least) tools are case-insensitive here.