Update user-defined-types/exercise.md (#1835)

Allow us to edit so we can complete the exercise :)

src/user-defined-types/exercise.md

@@ -12,7 +12,7 @@ This exercise only requires creating and populating data structures so that
 `main` runs without errors. The next part of the course will cover getting data
 out of these structures.
 
-```rust,should_panic
+```rust,editable,should_panic
 {{#include exercise.rs:event}}
     // TODO: add required variants
 }