fix a bug that causes the web UI to fail to display retry attempts

Technically this should really be `*job['args']` instead of `job['args']`. With this bug, in most case, the web UI still displays the retry attempts correctly simply because the default implementation of `retry_identifier(*args)` uses `args.join('-')` and, `['a'].join('-')` and `[['a']].join('-')` return the same string.