added C# Sinhala course tutorial series (#8565)

* added C# Sinhala course tutorial series

* Update courses/free-courses-si.md added suggested changes

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

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

courses/free-courses-si.md

@@ -5,6 +5,7 @@
 * [Java](#java)
 * [JavaScript](#javascript)
     * [React](#react)
+* [C#](#csharp)
 
 
 ### ASP.NET Core
@@ -32,3 +33,8 @@
 
 * [Fundamentals \| React JS in Sinhala](https://youtube.com/playlist?list=PLvvtf05eMZ2DpDyWwmAjEuicvVxx4vIYB) - Fiqri Ismail (YouTube)
 * [REACT | MERN CRUD App in Sinhala](https://youtube.com/playlist?list=PLtoqJbwHBeHzAooLCGOzYVE9mkAeCnT9y) - Haritha Weerathunga (YouTube)
+
+
+### <a id="csharp"></a>C\#
+
+* [C# Full Course in Sinhala](https://youtube.com/playlist?list=PLvvtf05eMZ2CXD2JdZgSBgyl13ODqHOkO) - Fiqri Ismail