Adds Learn4Elixir content (#10136)

* Adds Learn4Elixir repo link to

* Update books/free-programming-books-pt_BR.md

Co-authored-by: Eric Hellman <eric@hellman.net>

---------

Co-authored-by: Eric Hellman <eric@hellman.net>

books/free-programming-books-pt_BR.md

@@ -148,6 +148,7 @@
 ### Elixir
 
 * [Elixir DOJO](http://victorolinasc.github.io/elixir_dojo/dojo.html) - Victor Oliveira Nascimento (HTML)
+* [Learn4Elixir](https://github.com/Universidade-Livre/Learn4Elixir) - Universidade Brasileira Livre (Livebook)
 
 
 ### Engenharia de Software