typo in ch3.tex

[DrAnaximandre]

Hi, thanks for the repo it's very useful.

in ch3.tex, there is a small typo in ll71/72, as it should be

  p(X=1|\alpha, D_{1:3}) = \frac{\alpha_1+ 1}{\alpha + 3} \\
  p(X=1|\alpha, D_{1:4}) = \frac{\alpha_1 + 2}{\alpha + 4} \\

(alpha_1 instead of alpha_0)