docs(readme): Fix build status badge (#4559)

Both the link and the branch pointed to

README.md

@@ -6,7 +6,7 @@
 [![Crates.io](https://img.shields.io/crates/d/clap?style=flat-square)](https://crates.io/crates/clap)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue?style=flat-square)](LICENSE-APACHE)
 [![License](https://img.shields.io/badge/license-MIT-blue?style=flat-square)](LICENSE-MIT)
-[![Build Status](https://img.shields.io/github/workflow/status/clap-rs/clap/CI/staging?style=flat-square)](https://github.com/clap-rs/clap/actions/workflows/ci.yml?query=branch%3Astaging)
+[![Build Status](https://img.shields.io/github/actions/workflow/status/clap-rs/clap/ci.yml?branch=master&style=flat-square)](https://github.com/clap-rs/clap/actions/workflows/ci.yml?query=branch%3Amaster)
 [![Coverage Status](https://img.shields.io/coveralls/github/clap-rs/clap/master?style=flat-square)](https://coveralls.io/github/clap-rs/clap?branch=master)
 [![Contributors](https://img.shields.io/github/contributors/clap-rs/clap?style=flat-square)](https://github.com/clap-rs/clap/graphs/contributors)