Update sdk constraints

Author: gonuit

pubspec.yaml

@@ -6,7 +6,7 @@ issue_tracker: https://github.com/gonuit/flutter-custom-refresh-indicator/issues
 homepage: https://github.com/gonuit/flutter-custom-refresh-indicator
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
   flutter: ">=1.16.0"
 
 dependencies: