Update .filter function docs.

Callback passed to function `filter` get called with 2 arguments (`index` and `element`).

Readme.md

@@ -505,7 +505,7 @@ $('li').map(function(i, el) {
 //=> "apple orange pear"
 ```
 
-#### .filter( selector ) <br /> .filter( selection ) <br /> .filter( element ) <br /> .filter( function(index) )
+#### .filter( selector ) <br /> .filter( selection ) <br /> .filter( element ) <br /> .filter( function(index, element) )
 
 Iterates over a cheerio object, reducing the set of selector elements to those that match the selector or pass the function's test. When a Cheerio selection is specified, return only the elements contained in that selection. When an element is specified, return only that element (if it is contained in the original selection). If using the function method, the function is executed in the context of the selected element, so `this` refers to the current element.