Update to a hopefully more future proof FSF address

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 9101487..7eed642 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -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/>.
  */
 
 #include <qemu-common.h>