Add missing d\theta in posterior mean

The nominator in the posterior mean equation did not have d\theta.

stat-cookbook.tex

@@ -1209,7 +1209,7 @@ \section{Bayesian Inference}
   \item Kernel: part of a density that depends on $\theta$
   \item Posterior mean $\bar\theta_n
     = \int\theta f(\theta\giv x^n)\,d\theta
-    = \frac{\int\theta\Lln(\theta)f(\theta)}
+    = \frac{\int\theta\Lln(\theta)f(\theta)d\theta}
            {\int\Lln(\theta)f(\theta)\,d\theta}$
 \end{itemize}