📝 修复 rag/README.md 里的过时引用

- 从 app.graph.graph_builder 改为 app.main_graph.utils.main_graph_builder

backend/app/rag/README.md

@@ -321,14 +321,13 @@ def search_knowledge_base(query: str) -> str:
 ### 绑定到 LangGraph
 
 ```python
-from app.graph.graph_builder import GraphBuilder
+from app.main_graph.utils.main_graph_builder import build_react_main_graph
 
 # 将 RAG 工具添加到工具列表
 tools = AVAILABLE_TOOLS + [search_knowledge_base]
 
 # 构建图
-builder = GraphBuilder(llm, tools, tools_by_name)
-graph = builder.build().compile(checkpointer=checkpointer)
+graph = build_react_main_graph(llm, tools).compile(checkpointer=checkpointer)
 ```
 
 ## ⚙️ 环境配置