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-en.md

@@ -10,7 +10,7 @@
 * [Bash / Shell](#bash--shell)
 * [Bootstrap](#bootstrap)
 * [C](#c)
-* [C#](#c-sharp)
+* [C#](#csharp)
 * [C++](#cpp)
 * [Clojure](#clojure)
 * [Cloud Computing](#cloud-computing)
@@ -241,7 +241,7 @@
 * [The Arduino Platform and C Programming](https://www.coursera.org/learn/arduino-platform)
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [C# 101](https://channel9.msdn.com/Series/CSharp-101) - Scott Hanselman, Kendra Havens Microsoft.
 * [C# Programming All-in-One Tutorial Series (6 HOURS!)](https://www.youtube.com/watch?v=qOruiBrXlAw) - Caleb Curry (YouTube)
@@ -250,7 +250,7 @@
 * [Learn how to program: C# and .NET](https://www.learnhowtoprogram.com/c-and-net) - Epicodus Inc.
 
 
-<h3 id="cpp">C++</h3>
+### <a name="cpp"></a>C++
 
 * [C++ For Programmers](https://www.udacity.com/course/c-for-programmers--ud210) - Catherine Gamboa (Udacity)
 * [C++ Programming Tutorial for Beginners (For Absolute Beginners)](https://www.youtube.com/playlist?list=PLS1QulWo1RIYSyC6w2-rDssprPrEsgtVK) - ProgrammingKnowledge
@@ -746,7 +746,7 @@
 
 ### .NET
 
-> :information_source: See also &#8230; [C#](#c-sharp)
+> :information_source: See also &#8230; [C#](#csharp)
 
 
 ### Objective-C