fix compile warning

https://stackoverflow.com/questions/18039275/retained-block-property-does-not-copy-the-block-use-copy-attribute-instead

"Retain'ed block property does not copy the block - use copy attribute instead"

Not sure if this should be `copy` or `strong`.

ios/Classes/BarcodeScanPlugin.h

@@ -9,6 +9,6 @@
 
 @interface BarcodeScanPlugin : NSObject<FlutterPlugin, BarcodeScannerViewControllerDelegate>
 
-@property(nonatomic, retain) FlutterResult result;
+@property(nonatomic, copy) FlutterResult result;
 @property (nonatomic, assign) UIViewController *hostViewController;
 @end