dead link to web ui choices in pinned issue

The link in the pinned comment is not valid anymore (404).

> [...]. As of today, there are four web UIs listed in https://headscale.net/web-ui/, [...]
> 

 _Originally posted by @juanfont in [#234](https://github.com/juanfont/headscale/issues/234#issuecomment-2210629406)_

Working url: https://headscale.net/stable/ref/integration/web-ui/