Stefano Soccorsi pointed out that the one of the Kurtosis equations on page 37 is wrong. The rest of of it is correct, as is the final Kurtosis value. The typo was in the second equation below:

\begin{align*} E(Y^4) &= 3 E\left(\left(\omega+\alpha Y_{t-1}^2\right)^2\right) \ &= 3 \omega^2+ 6\alpha \omega E(Y^2)+3 \alpha^2 E(Y^4)\ &= 3 \omega^2+ 6\alpha \omega \frac{\omega}{1-\alpha}+3 \alpha^2 E(Y^4)\ \end{align*}