Fix: python_api_reference.md update dataset bug (#6527)

### What problem does this PR solve?

There is a small bug in the update dataset of this document. The return
type of rag_oobject.list_datasets is a list type, and the first item
should be taken as' ragflow_stdk.modules.dataset ' DataSet`， Adapt to
the update.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

docs/references/python_api_reference.md

 
 rag_object = RAGFlow(api_key="<YOUR_API_KEY>", base_url="http://<YOUR_BASE_URL>:9380")
 dataset = rag_object.list_datasets(name="kb_name")
+dataset = dataset[0]
 dataset.update({"embedding_model":"BAAI/bge-zh-v1.5", "chunk_method":"manual"})
 ```