Correct docs about which event is fired

fixes #2489

In the documentation it states a `failure` event is fired when a JSON
string could not be parsed, [but the fired event is `error`](https://github.com/mootools/mootools-core/blob/master/Source/Request/Request.JSON.js#L39). 

The [documentation for`onError`](https://github.com/mootools/mootools-core/blob/master/Docs/Request/Request.JSON.md#error) already describes that `error` is fired when a JSON string could not be parsed.

>#### error
Fired when the parsed JSON is not valid and the secure option is set.