update COPYING file

Licence in COPYING contains old fsf address.
Updated version was taken from:
http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt

Rpmlint is complaining about fedora's package of usbutils, because it contains incorrect fsf address (http://fedoraproject.org/wiki/Common_Rpmlint_issues#incorrect-fsf-address).