feat(books): update text

Author: dergigi

_includes/category-technical.md

@@ -2,11 +2,7 @@ If you would like to dig deeper or get your hands dirty with programming, these
 books are for you. Technical knowledge and skills in computer science,
 programming, and/or mathematics are beneficial.
 
-Mastering Bitcoin is [available for free on GitHub][mastering-bitcoin-free].
-Programming Bitcoin includes plenty of [programming
-exercises][programming-bitcoin-exercises], which are available for free on
-GitHub as well. Grokking Bitcoin is [available for free on
-GitHub][grokking-bitcoin-free] as well.
+Most of these books are open-source and thus available for free on GitHub.
 
 [mastering-bitcoin-free]: https://github.com/bitcoinbook/bitcoinbook
 [programming-bitcoin-exercises]: https://github.com/jimmysong/programmingbitcoin