Fixed broken links to reference to issue #6942 (#7945)

* Fixed broken link to Introduction to Parallel Computing in reference to Issue #6942

* Fixed broken link for 'The GLib/GTK+ Development Platform' in reference to Issue #6942

Co-authored-by: John Henson <jhenson@Johns-MacBook-Air.local>

books/free-programming-books-subjects.md

@@ -312,7 +312,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [GUI development with Rust and GTK 4](https://gtk-rs.org/gtk4-rs/stable/latest/book/) - Julian Hofer
 * [Programming with gtkmm 4](https://developer.gnome.org/gtkmm-tutorial/stable/)
 * [Search User Interfaces](http://searchuserinterfaces.com/book/) - Marti A. Hearst
-* [The GLib/GTK+ Development Platform](https://people.gnome.org/~swilmet/glib-gtk-dev-platform.pdf) - Sébastien Wilmet (PDF)
+* [The GLib/GTK+ Development Platform](https://informatique-libre.be/swilmet/glib-gtk-book/glib-gtk-dev-platform.pdf) - Sébastien Wilmet (PDF)
 * [Web Design Primer](https://pressbooks.library.ryerson.ca/webdesign/) - Richard Adams, Ahmed Sagarwala
 * [Web Style Guide Online](https://www.webstyleguide.com/wsg3/index.html) - Patrick J. Lynch, Sarah Horton
 
@@ -678,7 +678,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [High Performance Computing Training](https://web.archive.org/web/20170702124132/https://hpc.llnl.gov/training/tutorials) (LLNL materials)
 * [High-Performance Scientific Computing](https://andreask.cs.illinois.edu/Teaching/HPCFall2012) (class lectures and slides)
 * [Introduction to High-Performance Scientific Computing](http://pages.tacc.utexas.edu/~eijkhout/istc/istc.html) - Victor Eijkhout
-* [Introduction to Parallel Computing](https://computing.llnl.gov/tutorials/parallel_comp/) - Blaise Barney
+* [Introduction to Parallel Computing](https://hpc.llnl.gov/documentation/tutorials/introduction-parallel-computing-tutorial) - Blaise Barney
 * [Is Parallel Programming Hard, And, If So, What Can You Do About It?](https://www.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html) - Paul E. McKenney
 * [Programming on Parallel Machines; GPU, Multicore, Clusters and More](http://heather.cs.ucdavis.edu/parprocbook) - Norm Matloff
 Kerridge (PDF) (email address *requested*, not required)