Make license metadata SPDX compliant (#270)

Author: paolobarbolini

Cargo.toml

@@ -3,7 +3,7 @@ name = "io-uring"
 version = "0.6.3"
 authors = ["quininer <[email protected]>"]
 edition = "2018"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 repository = "https://github.com/tokio-rs/io-uring"
 homepage = "https://github.com/tokio-rs/io-uring"
 documentation = "https://docs.rs/io-uring"