A link to a book exchanged (#3344)

* Update free-programming-books-cs.md

a new book added

* Update free-programming-books-cs.md

* A link to a book changed

Exchanging a link for a new (more direct) one.

free-programming-books-cs.md

@@ -130,7 +130,7 @@
 
 ### Python
 
-* [Ponořme se do Pythonu 3](https://knihy.nic.cz) - Mark Pilgrim (PDF)
+* [Ponořme se do Pythonu 3](http://diveintopython3.py.cz/index.html) - Mark Pilgrim
 * [Učebnice jazyka Python](http://i.iinfo.cz/files/root/k/Ucebnice_jazyka_Python.pdf) (PDF)