Fix status badge (grpc#726)

Author: kevmoo

README.md

@@ -1,7 +1,7 @@
 The [Dart](https://www.dart.dev/) implementation of
 [gRPC](https://grpc.io/): A high performance, open source, general RPC framework that puts mobile and HTTP/2 first.
 
-[![CI status](https://github.com/grpc/grpc-dart/workflows/Dart/badge.svg)](https://github.com/grpc/grpc-dart/actions?query=workflow%3A%22Dart%22+branch%3Amaster)
+[![Dart](https://github.com/grpc/grpc-dart/actions/workflows/dart.yml/badge.svg)](https://github.com/grpc/grpc-dart/actions/workflows/dart.yml)
 [![pub package](https://img.shields.io/pub/v/grpc.svg)](https://pub.dev/packages/grpc)