Refine doc for Xinference (#322)

### What problem does this PR solve?

Issue link:#229

### Type of change

- [x] Documentation Update

docs/xinference.md

 <img src="https://github.com/infiniflow/ragflow/assets/12318111/bcbf4d7a-ade6-44c7-ad5f-0a92c8a73789" width="1300"/>
 </div>
 
-> Base URL: Enter the base URL where the Ollama service is accessible, like, http://<your-ollama-endpoint-domain>:11434
+> Base URL: Enter the base URL where the Xinference service is accessible, like, http://<your-xinference-endpoint-domain>:9997/v1
 
 - Use Xinference Models.