Rename 'collection' to 'items' in README

[on3iro]

The Neos 9 compatibility release contained an undocumented breaking change of the Flowpack.Listable:Collection prototype. The collections property changed from collection to items.

I was contemplating if it would be better to rename it back, to be more in line with the other prototypes. However this would introduce another breaking change, which is why I decided to only document the behavior for now.

Let me know what you think and if I should make any changes to my PR :)