Apply suggestion from @maartenba

Author: maartenba

src/content/docs/accesstokenmanagement/advanced/extensibility.md

@@ -78,6 +78,7 @@ public class CustomTokenRetriever(
 A custom token handler can be linked to your `HttpClient` by creating an `AccessTokenRequestHandler` and adding it to the request pipeline:
 
 ```csharp
+// Program.cs
 services.AddHttpClient<YourTypedHttpClient>()
     .AddDefaultAccessTokenResiliency()
     .AddHttpMessageHandler(provider =>