Fix copyright mention.

In fact Arnold G. Reinhold copyrighted each of the pages of the
`diceware.com` website. The idea as such was not copyrighted (and might
be uncopyrightable).

Mr. Reinhold, however, still owns a "diceware" trademark.

COPYRIGHT

-The diceware idea Copyright(C) 1995-2015, Arnold G. Reinhold,
-Cambridge, Massachusetts USA.
+diceware is a concept invented by Arnold G. Reinhold, Cambridge, Massachusetts
+USA.
 
 The English Diceware Wordlists used with this programme are licensed
 by Arnold G. Reinhold (files ``wordlists/wordlist_en_orig.asc``,