Clarified usage of `fetch` with two arguments

The description of `fetch` indicated that passing in a second argument would allow you to "set" a default argument. This surprised me - does this mean `fetch` will set the value of that key in the hash? I had to go into irb to check, and it does not. =)

I changed the wording from "set" to "use" to make it clearer that you're not mutating the hash when you return a default value..