[ci skip] Update free-programming-books link in getting started guide

https://github.com/vhf/free-programming-books seems to have moved to
https://github.com/EbookFoundation/free-programming-books

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/vhf/free-programming-books/blob/master/free-programming-books.md#ruby)
+* [List of Free Programming Books](https://github.com/EbookFoundation/free-programming-books/blob/master/free-programming-books.md#ruby)
 
 Be aware that some resources, while still excellent, cover versions of Ruby as old as
 1.6, and commonly 1.8, and will not include some syntax that you will see in day-to-day