Update data-flow-tutorials.md

Author: kromerm

articles/data-factory/data-flow-tutorials.md

@@ -68,6 +68,8 @@ As updates are constantly made to the product, some features have added or diffe
 
 [Dynamic Joins and Dynamic Lookups](https://youtu.be/CMOPPie9bXM)
 
+[Flatten transformation](https://youtu.be/VY2tFQJoAXE)
+
 ## Source and sink
 
 [Reading and writing JSONs](https://www.youtube.com/watch?v=yY5aB7Kdhjg)