[Chore][Doc] uses model id determined from OpenAI client (#17815)

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>

examples/online_serving/openai_chat_completion_structured_outputs.py

@@ -138,7 +138,7 @@ def main():
         api_key="-",
     )
 
-    model = "Qwen/Qwen2.5-3B-Instruct"
+    model = client.models.list().data[0].id
 
     print("Guided Choice Completion:")
     print(guided_choice_completion(client, model))

examples/online_serving/openai_chat_completion_structured_outputs_structural_tag.py

@@ -59,7 +59,7 @@ and San Francisco?
     }]
 
     response = client.chat.completions.create(
-        model="meta-llama/Llama-3.1-8B-Instruct",
+        model=client.models.list().data[0].id,
         messages=messages,
         response_format={
             "type":