update link for "Modern C" by Jens Gustedt, more authoritative (#3687)

* update link for "Modern C" by Jens Gustedt, more authoritative

* same as before, but remove trailing slash for the linter

free-programming-books.md

@@ -1009,7 +1009,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Functional C (1997)](https://research.utwente.nl/files/5128727/book.pdf) - Pieter H. Hartel, Henk Muller (PDF)
 * [Learn to Code With C - The MagPi Essentials](https://www.raspberrypi.org/magpi-issues/Essentials_C_v1.pdf) (PDF)
 * [Modeling with Data](https://ben.klemens.org/pdfs/gsl_stats.pdf) - Ben Klemens (PDF)
-* [Modern C](https://web.archive.org/web/20190818210107/http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf) - Jens Gustedt (PDF)
+* [Modern C](https://modernc.gforge.inria.fr) - 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
 * [The C book](http://publications.gbdirect.co.uk/c_book/)