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:

Start by locating the author-creation resolver or entry point and inspect how the authors array length is read. Compare the referenced changelog image with the current code, correct the typo, and verify that creating an author no longer raises "Cannot access 'author' before initialization".

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