Incoherency in parse_date function documentation

In Graylog 2.4 [functions documentation](http://docs.graylog.org/en/2.4/pages/pipelines/functions.html), there is incoherent information about `parse_date` function optional arguments order.

Under Overview, there is an example where `timezone` is the third parameter.
```
let new_date = parse_date(
to_string($message.transaction_date),
"yyyy-MM-dd HH:mm:ss",
to_string($message.transaction_timezone)
);
```

However, `parse_date` function reference shows that the `timezone` parameter is the *fourth* parameter
```
parse_date(value: string, pattern: string, [locale: string], [timezone: string])
```

It seems that `locale` parameter has been added in Graylog version 2.3 but the examples under Overview have not been updated accordingly.