Update Ubuntu Server guide to 16.04 (#1989)

free-programming-books.md

@@ -1401,7 +1401,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [The Linux System Administrator's Guide](http://www.tldp.org/LDP/sag/html/index.html)
 * [The Python GTK+ 3 Tutorial](http://python-gtk-3-tutorial.readthedocs.org/en/latest/)
 * [Ubuntu Pocket Guide and Reference](http://www.ubuntupocketguide.com/index_main.html)
-* [Ubuntu Server Guide](https://help.ubuntu.com/14.04/serverguide/serverguide.pdf) (PDF)
+* [Ubuntu Server Guide](https://help.ubuntu.com/16.04/serverguide/serverguide.pdf) (PDF)
 * [Upstart Intro, Cookbook and Best Practises](http://upstart.ubuntu.com/cookbook/)
 * [What Every Programmer Should Know About Memory](http://www.akkadia.org/drepper/cpumemory.pdf) (PDF)