Correct code sample to be compilable

Updated flag provider implementation to include additional using directives and modified the ShouldAddFlag method to be generic.

Author: glombek

17/umbraco-cms/extending/flag-providers.md

@@ -39,8 +39,12 @@ To create a flag provider, implement the `IFlagProvider` interface. There are tw
 An illustrative implementation is as follows:
 
 ```csharp
-using Umbraco.Cms.Api.Management.ViewModels;
 using Umbraco.Cms.Core;
+using Umbraco.Cms.Api.Management.Services.Flags;
+using Umbraco.Cms.Api.Management.ViewModels;
+using Umbraco.Cms.Api.Management.ViewModels.Document.Collection;
+using Umbraco.Cms.Api.Management.ViewModels.Document.Item;
+using Umbraco.Cms.Api.Management.ViewModels.Tree;
 
 internal class MyDocumentFlagProvider : IFlagProvider
 {
@@ -67,7 +71,7 @@ internal class MyDocumentFlagProvider : IFlagProvider
         return Task.CompletedTask;
     }
 
-    private bool ShouldAddFlag(TItem item) => return true; // Provide custom logic here.
+    private bool ShouldAddFlag<TItem>(TItem item) => true; // Provide custom logic here.
 }
 ```