http: --backup-directory expects a path, not a template of a path.

Remove this outdated mention.

caucase/http.py

@@ -354,7 +354,7 @@ def main(argv=None):
   )
   backup_group.add_argument(
     '--backup-directory',
-    help='Backup file path template. Backups will be periodically stored in '
+    help='Backup directory path. Backups will be periodically stored in '
     'given directory, encrypted with all certificates which are valid at the '
     'time of backup generation. Any one of the associated private keys can '
     'decypher it. If not set, no backup will be created.',