http: Fix comment.

It is not expiration which is disabled, but pruning from database.

caucase/http.py

@@ -442,7 +442,7 @@ def main(argv=None, until=utils.until):
       db_path=args.db,
       table_prefix='cau',
       max_csr_amount=args.user_max_csr,
-      # Effectively disables certificate expiration
+      # Effectively disables certificate pruning
       crt_keep_time=cau_crt_life_time,
       crt_read_keep_time=cau_crt_life_time,
       enforce_unique_key_id=True,