Update app.py

app.py

@@ -4,7 +4,7 @@ from llama_index.core import VectorStoreIndex, SimpleDirectoryReader, SummaryInd
 from llama_index.readers.web import SimpleWebPageReader
 from llama_index.llms.mistralai import MistralAI
 from llama_index.embeddings.mistralai import MistralAIEmbedding
-from llama_index.query_engine import RetrieverQueryEngine
+from llama_index.core.query_engine.retriever_query_engine import RetrieverQueryEngine
 
 title = "Gaia Mistral Chat RAG URL Demo copy"
 description = "Example of an assistant with Gradio, RAG from url and Mistral AI via its API"