fix typo

Author: avehtari

src/stan-users-guide/posterior-prediction.Rmd

@@ -48,7 +48,7 @@ p(\tilde{y} \mid y)
 \frac{1}{M} \sum_{m = 1}^M p(\tilde{y} \mid \theta^{(m)}).
 $$
 
-Computing directly using tis formula will lead to underflow in many
+Computing directly using this formula will lead to underflow in many
 situations, but the log posterior predictive density, $\log
 p(\tilde{y} \mid y)$ may be computed using the stable log sum of
 exponents function as