greek language code is el not gr (EbookFoundation#6070)

* correct bengali language code

* fix language code, replace spanish links with english

* lint

* greek language code is el not gr

Author: eshellman

README.md

@@ -46,7 +46,7 @@ Please read [CONTRIBUTING](CONTRIBUTING.md). If you're new to GitHub, [welcome](
 + [Finnish / suomi / suomen kieli](books/free-programming-books-fi.md)
 + [French / français](books/free-programming-books-fr.md)
 + [German / Deutsch](books/free-programming-books-de.md)
-+ [Greek / ελληνικά](books/free-programming-books-gr.md)
++ [Greek / ελληνικά](books/free-programming-books-el.md)
 + [Hebrew / עברית](books/free-programming-books-he.md)
 + [Hungarian / magyar / magyar nyelv](books/free-programming-books-hu.md)
 + [Indonesian / Bahasa Indonesia](books/free-programming-books-id.md)