docs: declare GPL3 license

refs #2672

Change-Id: I93e0cb6b7ff907d3f05d21aae2d21009efd3e2f0
diff --git a/core/common.hpp b/core/common.hpp
index a74b910..4085d68 100644
--- a/core/common.hpp
+++ b/core/common.hpp
@@ -8,7 +8,7 @@
  *                           Beijing Institute of Technology,
  *                           The University of Memphis.
  *
- * This file is part of ndn-tools (Named Data Networking Tools).
+ * This file is part of ndn-tools (Named Data Networking Essential Tools).
  * See AUTHORS.md for complete list of ndn-tools authors and contributors.
  *
  * ndn-tools is free software: you can redistribute it and/or modify it under the terms