Minor update

HengchengZhang · HengchengZhang · commit 00fb34fcc938 · 2023-08-01T10:21:37.000+10:00
diff --git a/lectures/intro_supply_demand.md b/lectures/intro_supply_demand.md
@@ -575,7 +575,7 @@ price, in line with the first fundamental welfare theorem.
 ```
 
 
-```{solution-start} isd_ex3
+```{solution-start} isd_ex4
 :class: dropdown
 ```
 
