[fix] debian/copyright to shut up lintian

Author: stef

debian/copyright

@@ -2,6 +2,10 @@ Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
 Upstream-Name: libsphinx
 Upstream-Contact: sphinx@ctrlc.hu
 Source: https://github.com/stef/libsphinx
+
 Files:     *
 Copyright: 2018-21 Stefan Marsiske <sphinx@ctrlc.hu>
 License:   LGPL-3+
+ On Debian systems, the full text of the GNU Lesser General
+ Public License version 3 can be found in the file
+ `/usr/share/common-licenses/LGPL-3'.