Fixes code example in docs

http://screencast.com/t/xHi1XoYcTJ
Using the word "context" makes it seems as if the `this` of the function will be the resource object, which is not the case.

Maybe [this line](https://github.com/angular/angular.js/blob/3df2ccae0f59da4ef7ff7ab03c086a69acb66555/src/ngResource/resource.js#L203) can have the following added to it: `[...] arguments, where the value is the resulting resource instance or collection object`?