Fix link to documentation

src/Validation/Validator.php

@@ -23,7 +23,7 @@ use Cake\Validation\ValidationSet;
  *
  * Implements ArrayAccess to easily modify rules in the set
  *
- * @link http://book.cakephp.org/2.0/en/data-validation.html
+ * @link http://book.cakephp.org/3.0/en/core-libraries/validation.html
  */
 class Validator implements \ArrayAccess, \IteratorAggregate, \Countable {