Fix small nits in HW2

[revalo]

Fixes three typos in HW2,

• Rename PixelCNN prior to Transformer prior in the Homework 2 $\LaTeX$ template.
• Remove stray parenthesis in Q2 (b), $N(z2; \mu_\theta(z1) + \Delta \mu_\theta(z1,x), \Sigma_\theta(z1) \ \textcolor{red}{\mathbf{)}} \ * \Delta \Sigma_\theta(z1,x))$ -> $N(z2; \mu_\theta(z1) + \Delta \mu_\theta(z1,x), \Sigma_\theta(z1) * \Delta \Sigma_\theta(z1,x))$
• In Q2 (b), $q_\theta(z|x)$ is renamed to $q_\theta(z1|x)$ as per IRL discussion with one of the TAs, please feel free to ignore this one, it's in a separate commit.