Correct documentation for `.formUpdate`

Documentation refers to a method `element.formRequest`. 
Such method [does not exist (jsfiddle)](http://jsfiddle.net/CHEuL/).

The method correct name is __[.formUpdate](https://github.com/mootools/mootools-more/blob/master/Source/Forms/Form.Request.js#L191)__

```js
console.log($('myForm').formUpdate);  // function (update, options){ ...
console.log($('myForm').formRequest); // undefined
```

fixes #254
fixes #1056