Update build status badge

Author: dtolnay

README.md

@@ -1,6 +1,6 @@
 # Serde JSON   [![Build Status]][travis] [![Latest Version]][crates.io] [![Rustc Version 1.36+]][rustc]
 
-[Build Status]: https://img.shields.io/github/workflow/status/serde-rs/json/CI/master
+[Build Status]: https://img.shields.io/github/actions/workflow/status/serde-rs/json/ci.yml?branch=master
 [travis]: https://github.com/serde-rs/json/actions?query=branch%3Amaster
 [Latest Version]: https://img.shields.io/crates/v/serde_json.svg
 [crates.io]: https://crates.io/crates/serde\_json