Incorrect documentation of the `split` method

The documentation of the `split` method suggest a return value of `(first, last, [middle])`, while it's actually `(first, [middle], last)`.

https://github.com/cloudhead/nonempty/blob/95d5cb131262b12bbe55366cbbd48096f9a05493/src/lib.rs#L532-L533

It can be fixed by either simply removing the "in that order" part, or by changing the listing to "first, middle and last elements".

I'd be happy creating a PR for fixing that, if you'd like.