Update LICENSE file to contain boilerplate project owner

As far as I understand the guidelines and looking at other projects (for example, Apache Kafka)
the LICENSE file must contain boilerplate ("Copyright [yyyy] [name of copyright owner]") whereas
other project files specific ("Copyright 2019 TiKV Project Authors").

Signed-off-by: dzmitry.huba@gmail.com

LICENSE

@@ -186,7 +186,7 @@ Apache License
       same "printed page" as the copyright notice for easier
       identification within third-party archives.
 
-   Copyright 2019 TiKV Project Authors.
+   Copyright [yyyy] [name of copyright owner]
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.