Update free-courses-en.md (#9396)

courses/free-courses-en.md

@@ -1115,6 +1115,7 @@
 
 * [Berkeley's CS 162: Operating Systems and Systems Programming](https://www.youtube.com/watch?v=feAOZuID1HM)
 * [Berkeley's CS 194: What is an Operating System?](http://www.infocobuild.com/education/audio-video-courses/computer-science/cs194-spring2013-berkeley.html)
+* [MIT 6.S081: Operating System Engineering(Fall 2020)](https://pdos.csail.mit.edu/6.S081/2020/schedule.html) - Frans Kaashoek, Robert Morris
 * [Operating System](https://www.youtube.com/playlist?list=PLBlnK6fEyqRiVhbXDGLXDk_OQAeuVcp2O) - Neso Academy