Update free-courses-en.md (#9399)

add youtube playlists for graph theory.

courses/free-courses-en.md

@@ -40,6 +40,7 @@
 * [Game Development](#game-development)
 * [Git](#git)
 * [Go](#go)
+* [Graph Theory](#graph-theory)
 * [Haskell](#haskell)
 * [HTML and CSS](#html-and-css)
     * [Bootstrap](#bootstrap)
@@ -671,6 +672,12 @@
 * [Let's go with golang](https://www.youtube.com/playlist?list=PLRAV69dS1uWQGDQoBYMZWKjzuhCaOnBpa) - Hitesh Choudhary
 
 
+### Graph Theory
+
+* [Graph Theory](https://youtube.com/playlist?list=PLztBpqftvzxXBhbYxoaZJmnZF6AUQr1mH) - Wrath of Math
+* [Graph Theory - DM](https://youtube.com/playlist?list=PLYrahs7hsYIQiSNxTfZndQz7jWPXsA1ur) - SCCI Labs IIT Ropar
+
+
 ### Haskell
 
 * [Advanced Functional Programming in Haskell](http://www.cs.nott.ac.uk/~pszgmh/afp.html) - Graham Hutton