Use SPDX license format

Author: atouchet

Cargo.toml

@@ -3,7 +3,7 @@ name = "backtrace"
 version = "0.3.64"
 authors = ["The Rust Project Developers"]
 build = "build.rs"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/rust-lang/backtrace-rs"
 homepage = "https://github.com/rust-lang/backtrace-rs"