Fix another typo.

lib/Cake/Model/Model.php

@@ -191,7 +191,7 @@ class Model extends Object {
  *
  * ### Valid keys in validations
  *
- * - `role`: String with method name, regular expression (started by slash) or array with method and parameters
+ * - `rule`: String with method name, regular expression (started by slash) or array with method and parameters
  * - `message`: String with the message or array if have multiple parameters. See http://php.net/sprintf
  * - `last`: Boolean value to indicate if continue validating the others rules if the current fail [Default: true]
  * - `required`: Boolean value to indicate if the field must be present on save