doc: update GPL license text

doc: update GPL license text

The Open Build Service/rpmlint flagged the outdated address in the
license text :-)

iptables.x86_64: W: incorrect-fsf-address
/usr/share/doc/packages/iptables/COPYING
The Free Software Foundation address in this file seems to be outdated
or misspelled.  Ask upstream to update the address, or if this is a
license file, possibly the entire file with a new copy available from
the FSF.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>