Update package.json

update version for package & editor

com.unity.uiwidgets/package.json

 {
 	"name": "com.unity.uiwidgets",
 	"displayName":"UIWidgets",
-	"version": "2.0.3-preview.1",
-	"unity": "2019.4",
+	"version": "2.0.4-preview.1",
+	"unity": "2020.3",
 	"description": "UIWidgets allows you to build beautiful cross-platform apps through Unity",
 	"dependencies": {
 	}