Use preferred syntax for the license

Cargo.toml

@@ -4,7 +4,7 @@ version = "3.0.0"
 authors = ["Michael Howell <michael@notriddle.com>"]
 description = "HTML Sanitization"
 keywords = [ "sanitization", "html", "security", "xss" ]
-license = "MIT / Apache-2.0"
+license = "MIT OR Apache-2.0"
 readme = "README.md"
 documentation = "https://docs.rs/ammonia/"
 repository = "https://github.com/rust-ammonia/ammonia"