docs: change license to MIT

I want more company to use `digs` without worries.

Author: azzamsa

Cargo.toml

@@ -6,7 +6,7 @@ categories = ["command-line-utilities"]
 edition = "2018"
 exclude = ["/README.md"]
 keywords = ["cli", "dns", "dig"]
-license = "GPL-3.0-only"
+license = "MIT"
 repository = "https://github.com/BiznetGIO/digs"
 rust-version = "1.76.0"
 description = "digs many at once"