aioble/examples: Change variable name to _ADV_INTERVAL_US.

Following 28136d8

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

micropython/bluetooth/aioble/examples/l2cap_file_server.py

@@ -33,7 +33,7 @@
 _CONTROL_CHARACTERISTIC_UUID = bluetooth.UUID("0492fcec-7194-11eb-9439-0242ac130003")
 
 # How frequently to send advertising beacons.
-_ADV_INTERVAL_MS = 250_000
+_ADV_INTERVAL_US = 250_000
 
 
 _COMMAND_SEND = const(0)