orgcard.tex: Use en-dash for copyright years

This matches what's in the Emacs tree (and generated automatically by
admin/update-copyright).

doc/orgcard.tex

@@ -15,7 +15,7 @@
 \pdflayout=(0l)
 
 % Nothing else needs to be changed below this line.
-% Copyright (C) 1987, 1993, 1996-1997, 2001-2021 Free Software
+% Copyright (C) 1987, 1993, 1996--1997, 2001--2021 Free Software
 % Foundation, Inc.
 
 % This document is free software: you can redistribute it and/or modify