Update header of the sample code

Author: justinyoo

docs/overview/developer_quickstart.cs

@@ -1,4 +1,5 @@
-// SAMPLE: Generate text from a simple prompt
+// SAMPLE: Generate text from a simple prompt through Responses API
+// PAGE: https://platform.openai.com/docs/overview
 // GUIDANCE: Instructions to run this code: https://aka.ms/oai/net/start
 #pragma warning disable OPENAI001