Add free course on Spark (#7273)

* Add free course on Spark

* Update order

* Update free-courses-en.md

* Fix and complete metadata

* Update courses/free-courses-en.md

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

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-en.md

@@ -957,6 +957,7 @@
 
 ### Spark
 
+* [Learn Spark](https://www.udacity.com/course/learn-spark-at-udacity--ud2002) -  David Drummond, Judit Lantos (Udacity)
 * [Spark Tutorial \| Spark Tutorial for Beginners \| Apache Spark Full Course - Learn Apache Spark 2020](https://www.youtube.com/watch?v=zC9cnh8rJd0) Great Learning