Outdated readme about `filedrop` event's `file` property

It's `files: File[]` now

> ### Respond to when a file is dropped on the element
>
> When a user has succesfully dropped a file on to the element, the
> element will emit a `filedrop` event. The `filedrop` event
> contains a `file` property which is a direct reference to the `File`
> that was dropped.
>
> ```HTML
> Drop a file here
>
>
> dropTarget.addEventListener('filedrop', (e) => {
> dropTarget.textContent = e.file.name;
> });
>
> ```
>
> Note: if more than one file is dropped on the element, only the first file
> will be included on the event.

Open the README and locate the section describing the `filedrop` event's `file` property and its example. Update the documentation to describe the `files: File[]` property and adjust the example and note to match the current event shape.

Written by the indexing model from the issue text.