aioble: Fix docs for subscribe (needs await).

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

Author: jimmo

micropython/bluetooth/aioble/README.md

@@ -118,7 +118,7 @@ temp_char = await temp_service.characteristic(_ENV_SENSE_TEMP_UUID)
 
 data = await temp_char.read(timeout_ms=1000)
 
-temp_char.subscribe(notify=True)
+await temp_char.subscribe(notify=True)
 while True:
  data = await temp_char.notified()
 ```