Update to a hopefully more future proof FSF address

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

nbd.h

diff --git a/nbd.h b/nbd.h
index eb77cce..5deac29 100644
--- a/nbd.h
+++ b/nbd.h
@@ -13,8 +13,7 @@
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
- *  along with this program; if not, write to the Free Software
- *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA  02110-1301 USA
+ *  along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
 #ifndef NBD_H