d/copyright: Replace FSF's old postal address with an URL.

Fixes: lintian: old-fsf-address-in-copyright-file
See-also: https://lintian.debian.org/tags/old-fsf-address-in-copyright-file.html

debian/copyright

@@ -26,9 +26,8 @@ License: LGPL-3
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU Library General Public License for more details.
  .
- You should have received a copy of the GNU Library General Public
- License along with this program; if not, write to Free Software Foundation, Inc.,
- 51 Franklin St, Fifth Floor, Boston,MA 02110-1301, USA.
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see https://www.gnu.org/licenses/.
  .
  On Debian systems, the complete text of the GNU Lesser General Public
  License version 3 can be found in `/usr/share/common-licenses/LGPL-3'.