Add themed logo to README (opensearch-project#988)

Signed-off-by: Miki <mehranb@amazon.com>

README.md

@@ -1,4 +1,4 @@
-<img src="https://opensearch.org/assets/brand/SVG/Logo/opensearch_logo_default.svg" height="64px"/>
+<img src="https://opensearch.org/assets/img/opensearch-logo-themed.svg" height="64px">
 
 - [Welcome!](#welcome)
 - [Project Resources](#project-resources)
@@ -34,4 +34,4 @@ This project is licensed under the [Apache v2.0 License](LICENSE.txt).
 
 ## Copyright
 
-Copyright 2020-2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+Copyright 2020-2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.