some makeup for the readme

Change one sentences of the readme in [Representing Singular Models And Collections](https://github.com/apotonick/representable/blob/master/README.md#representing-singular-models-and-collections)

From:
You can now parse collections to `Song`s.

To:
You can now parse collections to `Song` instances.

I think the '`Song`s' looks bad and like a misspelling.