Fixed link for two C++ resources (#5216)

Fixed broken links for following books in C++ section:
* Hands-On System Programming with C++
* How to make an Operating System

books/free-programming-books.md

@@ -1145,8 +1145,8 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Financial Numerical Recipes in C++](http://finance.bi.no/~bernt/gcc_prog/recipes/) - Bernt Arne Ødegaard
 * [Game Programming Patterns](http://gameprogrammingpatterns.com)
 * [Google's C++ Style Guide](https://google.github.io/styleguide/cppguide.html)
-* [Hands-On System Programming with C++](https://www.packtpub.com/free-ebooks/hands-system-programming-c) - Dr. Rian Quinn (Packt account *required*)
+* [Hands-On System Programming with C++](https://www.packtpub.com/free-ebook/hands-on-system-programming-with-c/9781789137880) - Dr. Rian Quinn (Packt account *required*)
-* [How to make an Operating System](https://www.gitbook.com/book/samypesse/how-to-create-an-operating-system/details) - Samy Pesse
+* [How to make an Operating System](https://samypesse.gitbook.io/how-to-create-an-operating-system/) - Samy Pesse
 * [How To Think Like a Computer Scientist: C++ Version](http://greenteapress.com/thinkcpp/index.html) - Allen B. Downey
 * [Introduction to Design Patterns in C++ with Qt 4](http://ptgmedia.pearsoncmg.com/images/9780131879058/downloads/0131879057_Ezust_book.pdf) - Alan Ezust, Paul Ezust (PDF)
 * [Joint Strike Fighter, C++ Coding Standards, by Bjarne Stroustrup](http://www.stroustrup.com/JSF-AV-rules.pdf) (PDF)