gordon.edu was back. unarchive and complete formats (#6731)

books/free-programming-books-subjects.md

@@ -458,7 +458,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Non-Uniform Random Variate Generation](http://luc.devroye.org/rnbookindex.html) - Luc Devroye (PDF)
 * [Notes on Diffy Qs](https://www.jirka.org/diffyqs/) - Jiří Lebl
 * [Number Theory](https://github.com/holdenlee/number-theory) - Holden Lee MIT
-* [Number Theory: In Context and Interactive](https://web.archive.org/web/20210711231452/https://math.gordon.edu/ntic/) - Karl-Dieter Crisman
+* [Number Theory: In Context and Interactive](https://math.gordon.edu/ntic/) - Karl-Dieter Crisman (HTML, PDF)
 * [Odds and Ends: Introducing Probability & Decision with a Visual Emphasis](https://jonathanweisberg.org/vip/) - Jonathan Weisberg
 * [Online Statistics Education](http://onlinestatbook.com) - David Lane
 * [OpenIntro Statistics](https://www.openintro.org/stat/textbook.php) - David M. Diez, Christopher D. Barr, Mine Çetinkaya-Rundel