Re: cal(1) has an outdated URL in the comment

> There is a diff below. I think it could be just deleted not updated
> because external documentations can be obsleted anytime.

Thanks. I think it's useful to have a link to an explanation since the
calculation is somewhat mystifying without it. If the link becomes dead
again, we could then use an archive.org (hopefully it will still exist).

So I committed your diff with the more specific link to the relevant
section: https://www.tondering.dk/claus/cal/week.php#calcweekno