docs(contributing): fix broken ref link to programming-playgrounds

Fix over EbookFoundation#5449 and commit 3c42c36

Author: davorpa

Diff for: CONTRIBUTING-es.md

@@ -247,4 +247,4 @@ Si es posible imprimirlo y conservar su esencia, no es un Tutorial Interactivo.
 [guidelines]: #pautas
 [in_process]: #in_process
 [markdown_guide]: https://guides.github.com/features/mastering-markdown/
-[programming_playgrounds_list]: (https://github.com/EbookFoundation/free-programming-books/blob/master/more/free-programming-playgrounds.md)
+[programming_playgrounds_list]: https://github.com/EbookFoundation/free-programming-books/blob/master/more/free-programming-playgrounds.md