Update README.md (#13)

Update the Github Actions link to filter to `main` branch

Author: mit-mit

README.md

@@ -1,5 +1,5 @@
 [![pub package](https://img.shields.io/pub/v/lints.svg)](https://pub.dev/packages/lints)
-[![Build Status](https://github.com/dart-lang/lints/workflows/validate/badge.svg)](https://github.com/dart-lang/lints/actions)
+[![Build Status](https://github.com/dart-lang/lints/workflows/validate/badge.svg)](https://github.com/dart-lang/lints/actions?query=branch%3Amain)
 
 This repo contains official Dart and Flutter lint rules.