Fix license field

Author: AntoineGagne

Cargo.toml

@@ -4,7 +4,7 @@ repository = "https://github.com/AntoineGagne/a-tree"
 description = "A dynamic data structure for efficiently indexing arbitrary boolean expressions"
 authors = ["Antoine Gagné <gagnantoine@gmail.com>"]
 readme = "README.md"
-license = "LICENSE"
+license-file = "LICENSE"
 version = "0.1.0"
 edition = "2021"
 build = "build.rs"