Fixed a format issue (#872)

### What problem does this PR solve?

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### Type of change

- [x] Documentation Update

docs/guides/llm_api_key_setup.md

@@ -9,7 +9,7 @@ You have two ways to input your LLM API key.
 
 ## Before Starting The System
 
-In **user_default_llm** of [service_conf.yaml](./docker/service_conf.yaml), you need to specify LLM factory and your own _API_KEY_. 
+In **user_default_llm** of [service_conf.yaml](https://github.com/infiniflow/ragflow/blob/main/docker/service_conf.yaml), you need to specify LLM factory and your own _API_KEY_. 
 RagFlow supports the flowing LLM factory, and with more coming in the pipeline:
 
 > [OpenAI](https://platform.openai.com/login?launch), [Tongyi-Qianwen](https://dashscope.console.aliyun.com/model),