Update LICENSE.txt text

LICENSE.txt

@@ -1,7 +1,9 @@
-Copyright (c) .NET Foundation. All rights reserved.
+Copyright (c) .NET Foundation and Contributors
+
+All rights reserved.
 
 Licensed under the Apache License, Version 2.0 (the "License"); you may not use
-these files except in compliance with the License. You may obtain a copy of the
+this file except in compliance with the License. You may obtain a copy of the
 License at
 
     http://www.apache.org/licenses/LICENSE-2.0