Added Formal Languages & Automata Theory Course (#7490)

* Added Formal Languages & Automata Theory Course

* Update courses/free-courses-en.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

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

courses/free-courses-en.md

@@ -1287,6 +1287,7 @@
 ### Theory
 
 * [Automata Theory](https://online.stanford.edu/courses/soe-ycsautomata-automata-theory)
+* [Formal Languages & Automata Theory](https://www.youtube.com/playlist?list=PLLvKknWU7N4zvTGcw9N2_7eZSTTkryb0U) - Lalit Vashistha
 * [Intro to Theoretical Computer Science](https://www.udacity.com/course/intro-to-theoretical-computer-science--cs313) (Udacity)