Correct documentation for `.formUpdate`

Documentation refers to a method `element.formRequest`. Such method
[does not exist](http://jsfiddle.net/CHEuL/).

The method 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
Merge: Update documentation for .formUpdate

Documentation refers to a method `element.formRequest()`.
Such method does not exist (http://jsfiddle.net/CHEuL/).

The method correct name is `.formUpdate()` as can be seen in the source code:

     Element.implement('formUpdate', function(update, options){

console.log($('myForm').formUpdate);  // function (update, options){ ...
console.log($('myForm').formRequest); // undefined

fixes #254
fixes #1056
closes #1254