Un-linkify "COPYING" in README.md

Since we're still planning to use this file verbatim in the PyPI
description, let's keep it simple for now.

Author: dranjan

README.md

@@ -42,4 +42,4 @@ Have questions? Feel free to ask in
 Copyright Darsh Ranjan and `plyfile` authors.
 
 This software is released under the terms of the GNU General Public
-License, version 3.  See the file [COPYING](COPYING) for details.
+License, version 3.  See the file `COPYING` for details.