Update build status badge

README.md

@@ -4,7 +4,7 @@ prettyplease::unparse
 [<img alt="github" src="https://img.shields.io/badge/github-dtolnay/prettyplease-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/prettyplease)
 [<img alt="crates.io" src="https://img.shields.io/crates/v/prettyplease.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/prettyplease)
 [<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-prettyplease-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/prettyplease)
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+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/prettyplease/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/prettyplease/actions?query=branch%3Amaster)
 
 A minimal `syn` syntax tree pretty-printer.