Added some spark tutorials (#6560)

* Update free-courses-en.md

* space

* space

* add attribution, index link

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

courses/free-courses-en.md

@@ -68,6 +68,7 @@
 * [Scala](#scala)
 * [Security](#security)
 * [Solidity](#solidity)
+* [Spark](#spark)
 * [Swift](#swift)
   * [Vapor](#vapor)
 * [Theory](#theory)
@@ -897,6 +898,11 @@
 * [Rust Tutorial](https://www.youtube.com/playlist?list=PLLqEtX6ql2EyPAZ1M2_C0GgVd4A-_L4_5) - Doug Milford
 
 
+### Spark
+
+* [Spark Tutorial | Spark Tutorial for Beginners | Apache Spark Full Course - Learn Apache Spark 2020](https://www.youtube.com/watch?v=zC9cnh8rJd0) Great Learning
+
+
 ### Scala
 
 * [Functional Programming Principles in Scala](https://www.coursera.org/course/progfun)