examples: use context manager in detection_callback.py

If we don't use a context manager, then we can get the following error
after a CTRL+C:

    Exception ignored in: <function BleakScannerWinRT.start.<locals>.<lambda> at 0x0000020C3A76EE80>
    Traceback (most recent call last):
    File "C:\Users\david\Documents\GitHub\bleak\bleak\backends\winrt\scanner.py", line 238, in <lambda>
        lambda s, e: event_loop.call_soon_threadsafe(self._received_handler, s, e)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\david\AppData\Local\Programs\Python\Python312\Lib\asyncio\base_events.py", line 840, in call_soon_threadsafe
        self._check_closed()
    File "C:\Users\david\AppData\Local\Programs\Python\Python312\Lib\asyncio\base_events.py", line 541, in _check_closed
        raise RuntimeError('Event loop is closed')
    RuntimeError: Event loop is closed

examples/detection_callback.py

@@ -30,9 +30,8 @@ async def main(args: argparse.Namespace):
 
     while True:
         logger.info("(re)starting scanner")
-        await scanner.start()
-        await asyncio.sleep(5.0)
-        await scanner.stop()
+        async with scanner:
+            await asyncio.sleep(5.0)
 
 
 if __name__ == "__main__":