Fix license field to be a valid SPDX expression

Author: johnbatty

Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Dawid Ciężarkiewicz <dpc@dpc.pw>"]
 description = "Asynchronous drain for slog-rs"
 keywords = ["slog", "logging", "log", "asynchronous"]
 categories = ["development-tools::debugging"]
-license = "MPL-2.0/MIT/Apache-2.0"
+license = "MPL-2.0 OR MIT OR Apache-2.0"
 documentation = "https://docs.rs/slog-async"
 homepage = "https://github.com/slog-rs/slog"
 repository = "https://github.com/slog-rs/async"