added Blockchain to index in free-courses-en.md (#7314)

courses/free-courses-en.md

@@ -8,6 +8,7 @@
 * [Assembly](#assembly)
 * [AutoIt](#autoit)
 * [Bash / Shell](#bash--shell)
+* [Blockchain](#blockchain)
 * [C](#c)
 * [C#](#csharp)
 * [C++](#cpp)