Add missing using directive for Elsa.Workflows.Attributes in GetDataActivity example

Co-authored-by: sfmskywalker <938393+sfmskywalker@users.noreply.github.com>

Author: Copilot

extensibility/custom-activities.md

@@ -758,6 +758,7 @@ Use `GetService<T>()` instead of `GetRequiredService<T>()` when a service is opt
 
 ```csharp
 using Elsa.Workflows;
+using Elsa.Workflows.Attributes;
 using Elsa.Workflows.Models;
 
 public class GetDataActivity : CodeActivity