[books] Move HgInit to books (#5475)

courses/free-courses-en.md

@@ -45,7 +45,6 @@
 * [Machine Learning](#machine-learning)
 * [Markdown](#markdown)
 * [Matlab](#matlab)
-* [Mercurial](#mercurial)
 * [Misc](#misc)
 * [.NET](#net)
 * [OCaml](#ocaml)
@@ -534,11 +533,6 @@
 * [MIT 18.S997 Introduction to MATLAB Programming](http://ocw.mit.edu/courses/mathematics/18-s997-introduction-to-matlab-programming-fall-2011/)
 
 
-### Mercurial
-
-* [Hg Init: a Mercurial Tutorial](https://hginit.github.io) - Joel Spolsky
-
-
 ### Misc
 
 * [Advanced Adobe XD (Web Design)](https://webdesign.tutsplus.com/courses/advanced-adobe-xd-for-everyone) - Adi Purdila