gnucash-docs maint: Replace COPYING file from GPL 3 to GPL 2.

[Frank H.Ellenberger]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/0ecd867b (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/40dab7df (commit)



commit 0ecd867b45584a31b4ecffa9bc04e9fc34e5f8b6
Author: TANIGUCHI Yasuaki <yasuakit at gmail.com>
Date:   Fri Sep 24 12:41:41 2021 +0900

    Replace COPYING file from GPL 3 to GPL 2.



Summary of changes:
 COPYING | 899 ++++++++++++++++++++--------------------------------------------
 1 file changed, 282 insertions(+), 617 deletions(-)