Minor fix on the Reference Guide

Hey folks,

It was brought to my attention that a small mistake went unnoticed on the Reference Guide. On page 18, the Overturn definition was wrong: 

OVERTURN
If you close the Spark and hit a ray on the final Spark Check

It should be the opposite. I rephrased the sentence and corrected it, and also added a definition for Closing Sparks. Here's how they read now:

CLOSING A SPARK
When you don't hit any rays and end up adding the fifth and final ray

OVERTURN
If you close the Spark and roll a 6 on the final Spark Check

I've also updated the print on demand version of the Reference Guide to reflect this change.

Cheers!

-- Cezar