Fix Python SDK example error. (#4263)

### What problem does this PR solve?

#4253
### Type of change

- [x] Documentation Update

docs/references/python_api_reference.md

 dataset = dataset[0]
 name = 'ragflow_test.txt'
 path = './test_data/ragflow_test.txt'
-rag_object.create_document(dataset, name=name, blob=open(path, "rb").read())
-doc = dataset.list_documents(name=name)
-doc = doc[0]
-dataset.async_parse_documents([doc.id])
-for c in rag_object.retrieve(question="What's ragflow?", 
-             dataset_ids=[dataset.id], document_ids=[doc.id], 
-             page=1, page_size=30, similarity_threshold=0.2, 
-             vector_similarity_weight=0.3,
-             top_k=1024
-             ):
-    print(c)
+documents =[{"displayed_name":"test_retrieve_chunks.txt","blob":open(path, "rb").read()}]
+docs = dataset.upload_documents(documents)
+doc = docs[0]
+doc.add_chunk(content="This is a chunk addition test")
+for c in rag_object.retrieve(dataset_ids=[dataset.id],document_ids=[doc.id]):
+  print(c)
 ```
 
 ---

sdk/python/ragflow_sdk/dataset.py

-#
-#  Copyright 2024 The InfiniFlow Authors. All Rights Reserved.
-#
-#  Licensed under the Apache License, Version 2.0 (the "License");
-#  you may not use this file except in compliance with the License.
-#  You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-#  Unless required by applicable law or agreed to in writing, software
-#  distributed under the License is distributed on an "AS IS" BASIS,
-#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#  See the License for the specific language governing permissions and
-#  limitations under the License.
-
-class DataSet:
-    def __init__(self, user_key, dataset_url, uuid, name):
-        self.user_key = user_key
-        self.dataset_url = dataset_url
-        self.uuid = uuid
-        self.name = name