nixos/man-db: trigger mandb generation by multi-user.target

default.target is an alias[1] to either multi-user.target or
graphical.target.  It makes a bit more sense to trigger mandb
generation by multi-user.target since the generation does not depend
on graphics.

[1]: man bootup

nixos/modules/misc/man-db.nix

@@ -136,7 +136,7 @@ in
             BindReadOnlyPaths = [ "/dev/null:/etc/man_db.conf" ]; # mandb will still read /etc/man_db.conf if it exists, even when setting -C path/to/config.conf
             ProtectSystem = "strict";
           };
-          wantedBy = [ "default.target" ];
+          wantedBy = [ "multi-user.target" ];
         };
       })
     ]