Update reference link for [ZFS/en] ZFS tuning guide

The ZFS article page is awesome, with plenty of clear explanations and a handful of resources at the end. The ZFS article has a link to the ZFS tuning guide from the OpenZFS guide. But it seems the page has been moved and is out of date on the website.

The new OpenZFS tuning guide is referenced here:
(https://openzfs.github.io/openzfs-docs/Performance%20and%20Tuning/Workload%20Tuning.html)

However, the learnxinyminutes references it to the old page. Please add it as an update.