fix beta distribution expression

Author: yixuan

src/stan-users-guide/problematic-posteriors.qmd

@@ -467,7 +467,7 @@ p(\phi \mid y)
 \end{align*}
 
 If $N = 9$ and each $y_n = 1$, the posterior is
-$\textsf{beta}(\phi \mid 9.5,0,5)$.  This posterior is unbounded as $\phi
+$\textsf{beta}(\phi \mid 9.5,0.5)$.  This posterior is unbounded as $\phi
 \rightarrow 1$.  Nevertheless, the posterior is proper, and although
 there is no posterior mode, the posterior mean is well-defined with a
 value of exactly 0.95.