Fixed Markdown error in free-programming-books-en.md (EbookFoundation#6200)

Co-authored-by: Brian Choromanski <[email protected]>

Author: Choromanski

books/free-programming-books-en.md

@@ -6,5 +6,5 @@
 ### All
 
 * [English, By Programming Language](free-programming-books-langs.md)
-  [English, By Subject](free-programming-books-subjects.md)
+* [English, By Subject](free-programming-books-subjects.md)
   (The list of books in English is here for historical reasons.)