update example

pubspec.yaml

@@ -4,7 +4,7 @@ version: 3.1.0-nullsafety.1
 homepage: https://github.com/JarvanMo/fluwx
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.1.0 <3.0.0"
   flutter: ">=1.12.0 <2.0.0"
 
 dependencies: