[Doc] Fix outdated source reference comment in anthropic/serving.py (vllm-project#40189)

Signed-off-by: z1ying <tzzying@outlook.com>

Author: z1ying

vllm/entrypoints/anthropic/serving.py

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 # Adapted from
-# https://github.com/vllm/vllm/entrypoints/openai/serving_chat.py
+# https://github.com/vllm-project/vllm/blob/main/vllm/entrypoints/openai/chat_completion/serving.py
 
 """Anthropic Messages API serving handler"""