remove SellableInterface from documentation

[gondo]

documentation is still referring to SellableInterface even though this was obviously removed.
https://github.com/Sylius/Sylius-Docs/blob/master/bundles/SyliusOrderBundle/installation.rst

im still new to this bundle so im not confident enough to fix this by myself

[atheleron]

I'm stuck on this one to don't know how to fix this there is no new docs and they removed SellableInterface in version 1.0 i read somewhere that SellableInterface is changed to OrderInterface but not sure if that works please update documentation!!

[gondo]

@atheleron just look at the code, here are all classes and interfaces:
https://github.com/Sylius/Sylius/tree/master/src/Sylius/Bundle/OrderBundle/Model