Update free-courses-en.md (#10808)

* Update free-courses-en.md

* Update courses/free-courses-en.md

Co-authored-by: Leithen <lcrider@cvent.com>

* Update courses/free-courses-en.md

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

---------

Co-authored-by: Leithen <lcrider@cvent.com>
Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-en.md

@@ -1632,6 +1632,7 @@
 * [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)
+* [Theory of Computation](https://www.youtube.com/playlist?list=PLyqSpQzTE6M9-v4V62bCygAVYGluaijyo) - Subrahmanyam Kalyanasundaram
 * [Theory of Computation & Automata Theory](https://www.youtube.com/playlist?list=PLBlnK6fEyqRgp46KUv4ZY69yXmpwKOIev) - Neso Academy