README.md: fix reference to COPYING and COPYING.LESSER

README.md

@@ -20,7 +20,7 @@ website http://graph-tool.skewed.de.
 
 graph-tool is free software, you can redistribute it and/or modify it
 under the terms of the GNU Lesser General Public License, version 3 or
-above. See LICENSE and LICENSE.LESSER for details.
+above. See COPYING and COPYING.LESSER for details.
 
 This program is distributed in the hope that it will be useful, but
 WITHOUT ANY WARRANTY; without even the implied warranty of