Minor improvements to the REST principles chapter

Well written summary!
While reading through, I made some clarifications (from my POV).

In addendum, see [a post of mine clarifying REST concepts](http://hyperrest.github.io/2013-03-30-rest-api-concepts/#content). To a great extent, I think it still is a good analogy (1.5y after).
I don't think there's any need to change 'system' into 'architecture'.