Removed a PHP paid book

The "Official Documentation (Offline Version)" Laravel book isn't free.

free-programming-books.md

@@ -1622,7 +1622,6 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
   * [Laravel 4 Official Documentation. Synced Daily](https://leanpub.com/laravel-5)
   * [Laravel: Code Bright](http://daylerees.com/codebright)
   * [Laravel Collections Unraveled](https://leanpub.com/laravelcollectionsunraveled)
-  * [Official Documentation (Offline Version)](https://leanpub.com/l4-offline-doc)
 * [PHP 5 Power Programming](http://www.informit.com/content/images/013147149X/downloads/013147149X_book.pdf) (PDF)
 * [PHP Best Practices](https://phpbestpractices.org)
 * [PHP Internals Book](http://www.phpinternalsbook.com)