Rename COPYING.md to COPYING.txt

Author: hollasch

main.cc

@@ -5,7 +5,7 @@
 // neighboring rights to this software to the public domain worldwide. This software is distributed
 // without any warranty.
 //
-// You should have received a copy (see file COPYING.md) of the CC0 Public Domain Dedication along
+// You should have received a copy (see file COPYING.txt) of the CC0 Public Domain Dedication along
 // with this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //==================================================================================================