Fix license field to be a valid SPDX expression

Author: johnbatty

Cargo.toml

@@ -4,7 +4,7 @@ version = "4.3.0"
 authors = ["Dawid Ciężarkiewicz <[email protected]>"]
 description = "Logging scopes for slog-rs"
 keywords = ["slog", "logging", "slog", "log"]
-license = "MPL-2.0/MIT/Apache-2.0"
+license = "MPL-2.0 OR MIT OR Apache-2.0"
 documentation = "https://docs.rs/slog-scope"
 homepage = "https://github.com/slog-rs/slog"
 repository = "https://github.com/slog-rs/scope"