Specify license as just MIT

Simplify the SPDX string to the user-facing version to make it easier for
users and tooling to understand. Contributions must still be `MIT OR Apache-2.0`. 

[ add commit body with context - Trevor ]

Author: Manishearth

Cargo.toml

@@ -4,7 +4,7 @@ categories = ["no-std"]
 description = "libm in pure Rust"
 documentation = "https://docs.rs/libm"
 keywords = ["libm", "math"]
-license = "MIT AND (MIT OR Apache-2.0)"
+license = "MIT"
 name = "libm"
 readme = "README.md"
 repository = "https://github.com/rust-lang/libm"