Replace the link for the book "Programming in Oberon". (#2675)

It pointed to a tutorial and not to the book.

free-programming-books.md

@@ -1656,7 +1656,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 
 * [Algorithms and Data-Structures](http://www.ethoberon.ethz.ch/WirthPubl/AD.pdf) - Niklaus Wirth (PDF)
 * [Object-Oriented Programming in Oberon-2](http://ssw.jku.at/Research/Books/Oberon2.pdf) - Hanspeter Mössenböck (PDF)
-* [Programming in Oberon](http://www.ethoberon.ethz.ch/WirthPubl/ProgInOberon.pdf) - Niklaus Wirth (PDF)
+* [Programming in Oberon](https://www.inf.ethz.ch/personal/wirth/ProgInOberonWR.pdf) - Niklaus Wirth (PDF)
 
 
 ### Objective-C