Update license field following SPDX 2.1 license expression standard

The new recommendation is to follow the SPDX 2.1 standard.
This allows automatic license verification software to work properly.

Reference: https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

Cargo.toml

@@ -4,7 +4,7 @@ categories = ["memory-management", "rust-patterns", "no-std"]
 description = "A fast bump allocation arena for Rust."
 documentation = "https://docs.rs/bumpalo"
 edition = "2021"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 name = "bumpalo"
 readme = "README.md"
 repository = "https://github.com/fitzgen/bumpalo"