debian/copyright: drop obsolete FSF street address

The 'You should have received a copy ... write to the Free
Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301 USA' paragraph flags the lintian tag
'old-fsf-address-in-copyright-file'. Debian prefers either no
address at all or an https://www.gnu.org/licenses/ reference;
in this file /usr/share/common-licenses/LGPL-2.1 is already
cited a few lines below, so the address is redundant. Replace
with the gnu.org URL per current FSF boilerplate.

Pre-existing text, no change to the licence terms.

Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>

debian/copyright

@@ -18,8 +18,7 @@ License: LGPL-2.1
  License for more details.
  .
  You should have received a copy of the GNU Lesser General Public License
- along with this library; if not, write to the Free Software Foundation, Inc.,
- 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ along with this library; if not, see <https://www.gnu.org/licenses/>.
  .
  On Debian systems, the full text of the GNU Lesser General Public License
  version 2.1 can be found in the file "/usr/share/common-licenses/LGPL-2.1".