docs(debian/copyright): point to FSF website instead of (outdated) FSF address

The outdated FSF address in the debian/copyright file was pointed out to me by lintian on a local sbuild run with this config.

I'm not quite sure whether this is the correct way to go about fixing the underlying "issue", but

• the same formulation ("if not, visit the http://fsf.org website.") exists for a couple of licences in the file already,
• lintian does not complain anymore with this change.

Maybe using https:// would be preferable to http://?

If I am completely misguided with this MR: apologies.