chore: updated sdk constraints for example (#60)

Author: alestiago

example/pubspec.yaml

@@ -4,7 +4,7 @@ version: 0.0.1
 homepage: https://github.com/VeryGoodOpenSource/mockingjay
 
 environment:
-  sdk: ">=2.19.0 <3.0.0"
+  sdk: ">=2.18.0 <4.0.0"
 
 dependencies:
   bloc: ^8.0.0