update drop-off for last exercise

HumphreyYang · HumphreyYang · commit 5dd46becaac5 · 2023-02-27T13:54:29.000+11:00
diff --git a/lectures/markov_chains.md b/lectures/markov_chains.md
@@ -1550,7 +1550,9 @@ power $P^k$ for all $k \in \mathbb N$.
 
 
 ```{solution-start} mc_ex_pk
+:class: dropdown
 ```
+
 Suppose that $P$ is stochastic and, moreover, that $P^k$ is
 stochastic for some integer $k$.
 
