pure-ftpd: fix meta.license

There is no licenses.bsd. Consulting the source reveals that it
uses the language of the ISC license (which is supposed to be equivalent to
the simplified BSD license).

pkgs/servers/ftp/pure-ftpd/default.nix

@@ -11,7 +11,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "A free, secure, production-quality and standard-conformant FTP server";
     homepage = https://www.pureftpd.org;
-    license = licenses.bsd;
+    license = licenses.isc; # with some parts covered by BSD3(?)
     maintainers = [ maintainers.lethalman ];
   };