correct icon license, we use Elementary icons, not Silk anymore

COPYING-README

@@ -8,7 +8,7 @@ Licensing of components:
 * MDB2: BSD style custom
 * User: AGPL
 * XML/RPC: MIT / PHP
-* Silk icons: Creative Commons Attribution 
+* Elementary filetype icons: GPL v3+
 All unmodified files from these and other sources retain their original copyright
 and license notices: see the relevant individual files.