Fix typo in class structure identifier

Author: harmn

week3/gene_expression.qmd

@@ -69,7 +69,7 @@ This returns a list (actually a `SimpleList`) of data structures, in this case o
   se <- exp[[1]]
 ```
 
-::: {#exr-class structure}
+::: {#exr-class_structure}
 To learn more about the resulting data in `se` use the `class()` function to see what kind we are dealing with.
 
 ```{r}