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

books/free-programming-books-zh.md

@@ -28,7 +28,7 @@
   * [Assembly](#assembly)
   * [AWK](#awk)
   * [C](#c)
-  * [C#](#c-sharp)
+  * [C#](#csharp)
   * [C++](#cpp)
   * [CoffeeScript](#coffeescript)
   * [Dart](#dart)
@@ -334,12 +334,12 @@
 * [Linux C 编程一站式学习](http://docs.linuxtone.org/ebooks/C&CPP/c/)
 
 
-### C Sharp
+### <a name="csharp"></a>C\#
 
 * [精通C#(第6版)](http://book.douban.com/subject/24827879/)
 
 
-<h3 id="cpp">C++</h3>
+### <a name="cpp"></a>C++
 
 * [100个gcc小技巧](https://github.com/hellogcc/100-gcc-tips/blob/master/src/index.md)
 * [100个gdb小技巧](https://github.com/hellogcc/100-gdb-tips/blob/master/src/index.md)