[Misc] Add SPDX-FileCopyrightText (#19100)

Signed-off-by: simon-mo <simon.mo@hey.com>

examples/online_serving/gradio_openai_chatbot_webserver.py

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """Example for starting a Gradio OpenAI Chatbot Webserver
 Start vLLM API server:
     vllm serve meta-llama/Llama-2-7b-chat-hf