fix lintian override for non-standard-dir-perm

debian/backupninja.lintian-overrides

 # Policy §10.9 wants 0755 or 2770 but since secrets could be stored there
 # we want this only accessed by root, and optionally by the "admingroup"
 # defined in backupninja.conf
-non-standard-dir-perm etc/backup.d/ 2770 != 0755
+non-standard-dir-perm 2770 != 0755 [etc/backup.d/]
 # The first occurrence is for the handler, the 2nd is for the package
 spelling-error-in-description rsync rsync (duplicate word) rsync
 spelling-error-in-description restic restic (duplicate word) restic