[flutter] Fix multiply blend mode

See https://esotericsoftware.com/forum/d/25369-spine-flutter-slot%E6%B7%B7%E5%90%88%E6%A8%A1%E5%BC%8F%E5%90%8Ealpha%E4%B8%A2%E5%A4%B1/2

spine-flutter/lib/spine_flutter.dart

@@ -551,7 +551,7 @@ class SkeletonData {
 enum BlendMode {
   normal(0, rendering.BlendMode.srcOver),
   additive(1, rendering.BlendMode.plus),
-  multiply(2, rendering.BlendMode.modulate),
+  multiply(2, rendering.BlendMode.multiply),
   screen(3, rendering.BlendMode.screen);
 
   final int value;