Fix the link for Modern C (#7068)

* Fix the link for Modern C

The link is not valid for it.

* Fix for new link

Fix to https://gustedt.gitlabpages.inria.fr/modern-c/

books/free-programming-books-langs.md

@@ -445,7 +445,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Learning GNU C](https://download-mirror.savannah.gnu.org/releases/c-prog-book/learning_gnu_c.pdf) - Ciaran O’Riordan (PDF)
 * [Let us C](https://web.archive.org/web/20211006163041/http://pdvpmtasgaon.edu.in/uploads/dptcomputer/Let%20us%20c%20-%20yashwantkanetkar.pdf) - Yashavant Kanetkar (PDF) *(:card_file_box: archived)*
 * [Modeling with Data](https://ben.klemens.org/pdfs/gsl_stats.pdf) - Ben Klemens (PDF)
-* [Modern C](https://modernc.gforge.inria.fr) - Jens Gustedt (PDF)
+* [Modern C](https://gustedt.gitlabpages.inria.fr/modern-c/) - Jens Gustedt (PDF)
 * [Object-Oriented Programming With ANSI-C](http://www.planetpdf.com/codecuts/pdfs/ooc.pdf) (PDF)
 * [Programming in C](http://ee.hawaii.edu/~tep/EE160/Book/PDF/) - Bharat Kinariwala & Tep Dobry
 * [Programming in C](https://www.freetechbooks.com/programming-in-c-t1337.html) - Kishori Mundargi