Fix dual license in Cargo.toml

Use 'MIT OR Apache-2.0' SPDX expression per Rust API guidelines.

Author: asaaki

Cargo.toml

@@ -3,7 +3,7 @@ name = "peas"
 version = "0.2.0"
 edition = "2024"
 description = "A CLI-based, flat-file issue tracker for humans and robots"
-license = "MIT"
+license = "MIT OR Apache-2.0"
 repository = "https://github.com/asaaki/peas"
 homepage = "https://github.com/asaaki/peas"
 documentation = "https://github.com/asaaki/peas#readme"