Commonize license identifier in DESCRIPTION

Change-Id: Ic07957a6e7917d000dcbcb947cf42d9885d002c4
diff --git a/DESCRIPTION b/DESCRIPTION
index 9b0b19d..91bbd45 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -5,7 +5,7 @@
 Author: KorAP-Team @ IDS Mannheim
 Maintainer: KorAP-Team <korap@ids-mannheim.de>
 Description: A simple package that makes KorAP's web serice API accessible from R.
-License: BSD-2
+License: BSD_2_clause + file LICENSE
 Encoding: UTF-8
 LazyData: false
 RoxygenNote: 6.1.1