Fix tyop in speaker notes (#2627)

Author: randomPoison

src/error-handling/exercise.md

@@ -18,7 +18,7 @@ to use as the error type for `eval`.
 {{#include exercise.rs:tests}}
 ```
 
-<detail>
+<details>
 
 - The starting code here isn't exactly the same as the previous exercise's
   solution: We've added in an explicit panic to show students where the error