Fix MockRepository example in docs (#19)

* Add Default implementation in example

* Derive Default for MockRepository in docs

Replace the manual implementation with the derive attribute in the rstest fixture example. This keeps the code concise and ensures the example compiles.

Author: leynos

docs/rust-testing-with-rstest-fixtures.md

@@ -261,6 +261,7 @@ trait Repository {
     fn get_item_name(&self, id: &str) -> Option<String>;
 }
 
+#[derive(Default)]
 struct MockRepository {
     data: HashMap<String, String>,
 }