Update link to "Mastering PixInsight"

The official channel to buy this amazing book is author's website.
Unfortunately, the site is currently down and it's not clear for
how long.

As a workaround, I updated the link to point to a discussion on
the CloudyNights forums.

README.md

@@ -46,7 +46,7 @@ Contributions are welcome. Please read the [contributing guideline](CONTRIBUTING
 
 - [Astrophotography is Easy!: Basics for Beginners](https://www.goodreads.com/book/show/56053741-astrophotography-is-easy)
 - [Inside PixInsight](https://www.goodreads.com/book/show/42603255-inside-pixinsight)
-- [Mastering PixInsight (and the Art of Astroimage Processing)](http://www.deepskycolors.com/mastering-pixinsight.html)
+- [Mastering PixInsight (and the Art of Astroimage Processing)](https://www.cloudynights.com/topic/735971-mastering-pixinsight-by-rogelio-andreo/)
 - [The Astrophotography Manual: A Practical and Scientific Approach to Deep Sky Imaging](https://www.goodreads.com/book/show/37301142-the-astrophotography-manual)
 - [The Deep-Sky Imaging Primer](https://www.goodreads.com/book/show/61296649-the-deep-sky-imaging-primer-third-edition)