Fix a type error in platform_channel_swift example (#17354)

examples/platform_channel_swift/lib/main.dart

@@ -47,7 +47,7 @@ class _PlatformChannelState extends State<PlatformChannel> {
     });
   }
 
-  void _onError(PlatformException error) {
+  void _onError(Object error) {
     setState(() {
       _chargingStatus = 'Battery status: unknown.';
     });