Update license to MIT

Signed-off-by: Ming-Wei Shih <mishih@microsoft.com>

src/parser.rs

@@ -1,5 +1,5 @@
-// Copyright (c) Rego-Rs Authors.
-// Licensed under the Apache 2.0 license.
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
 
 use crate::ast::*;
 use crate::lexer::*;