Bug due to typo, can't create an author.

Hey there, super neat repo and a nice introduction to the topic of GraphQL. I was actually using this to introduce an intern to the concept. It's been very helpful to quickly put a presentation together. I did notice one small bug. Due to a typo, when adding an author, you're not actually getting the length from the authors array. Instead, you've got "author" which throws the error "Cannot access 'author' before initialization".

Here's the whole changelog to fix it: