fix: Use Markdown instead of HTML markup for H3 & H4 headings (#6689)

* fix: Markdown markup for `C++` heading

* fix: Markdown markup for `Дизайн и Aрхитектура` heading

* fix: Markdown H4 markup for `Next.js` heading

* fix: Markdown H3 markup for `C#` heading

courses/free-courses-ko.md

@@ -5,7 +5,7 @@
 * [Arduino](#arduino)
 * [ASP.NET](#asp.net)
 * [C/C++](#c/c++)
-* [C#](#c-sharp)
+* [C#](#csharp)
 * [Circuit](#circuit)
 * [Deep Learning](#deep-learning)
 * [Flutter](#flutter)
@@ -80,7 +80,7 @@
 * [C언어 코딩도장](https://dojang.io/course/view.php?id=2)
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [예제로 배우는 C# 강좌](https://www.youtube.com/playlist?list=PL4PkN2EXiuVF3Xl0HNVMdY-_kMM3oyBds)