Update license badge to match Cargo.toml

Author: taiki-e

README.md

@@ -2,7 +2,7 @@
 
 [![Build](https://github.com/smol-rs/async-io/workflows/Build%20and%20test/badge.svg)](
 https://github.com/smol-rs/async-io/actions)
-[![License](https://img.shields.io/badge/license-MIT%2FApache--2.0-blue.svg)](
+[![License](https://img.shields.io/badge/license-Apache--2.0_OR_MIT-blue.svg)](
 https://github.com/smol-rs/async-io)
 [![Cargo](https://img.shields.io/crates/v/async-io.svg)](
 https://crates.io/crates/async-io)