Fix broken link for free programming books [ci-skip]

The file was moved by EbookFoundation/free-programming-books#5199.

Fixes rails#40977.

guides/source/getting_started.md

@@ -28,7 +28,7 @@ curve diving straight into Rails. There are several curated lists of online reso
 for learning Ruby:
 
 * [Official Ruby Programming Language website](https://www.ruby-lang.org/en/documentation/)
-* [List of Free Programming Books](https://github.com/EbookFoundation/free-programming-books/blob/master/free-programming-books.md#ruby)
+* [List of Free Programming Books](https://github.com/EbookFoundation/free-programming-books/blob/master/books/free-programming-books.md#ruby)
 
 Be aware that some resources, while still excellent, cover older versions of
 Ruby, and may not include some syntax that you will see in day-to-day