Update app.py

Adjuste error with obsolete lbrary

app.py

@@ -8,6 +8,7 @@ import math
 import json
 import unicodedata
 from langgraph.prebuilt import create_react_agent
+from langchain_core.messages import SystemMessage
 from langchain_huggingface import ChatHuggingFace, HuggingFaceEndpoint
 from langchain_core.tools import tool
 from langchain_community.tools import DuckDuckGoSearchRun
@@ -144,7 +145,7 @@ class BasicAgent:
         self.agent = create_react_agent(
             model=llm,
             tools=tools,
-            state_modifier=SYSTEM_PROMPT,
+            prompt=SystemMessage(content=SYSTEM_PROMPT),
         )
         print("Agent ready.")