Use SPDX license format (#278)

Cargo.toml

@@ -3,7 +3,7 @@ name = "smallvec"
 version = "1.8.0"
 edition = "2018"
 authors = ["The Servo Project Developers"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 repository = "https://github.com/servo/rust-smallvec"
 description = "'Small vector' optimization: store up to a small number of items on the stack"
 keywords = ["small", "vec", "vector", "stack", "no_std"]