broken sql pdf link replaced (#2088)

free-programming-books.md

@@ -1520,7 +1520,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 ### MySQL
 
 * [MySQL Essentials](http://www.techotopia.com/index.php/MySQL_Essentials)
-* [MySQL Tutorial Excerpt](http://downloads.mysql.com/docs/mysql-tutorial-excerpt-5.1-en.pdf) (PDF)
+* [MySQL Tutorial Excerpt](http://downloads.mysql.com/docs/mysql-tutorial-excerpt-5.5-en.pdf) (PDF)
 
 
 ### Neo4J