update license identifier in cargo.toml (#6)

* update license identifier in cargo.toml

Cargo.toml

@@ -4,7 +4,7 @@ authors = ["longjin <longjin@DragonOS.org>"]
 version = "0.1.0"
 edition = "2021"
 description = "DragonOS Application Development Kit\nDragonOS应用开发工具"
-license = "GPLv2"
+license = "GPL-2.0-only"
 repository = "https://github.com/DragonOS-Community/DADK.git"
 readme = "README.md"