fix example to use tool_plain

Author: dsfaccini

pydantic_ai_slim/pydantic_ai/agent/__init__.py

@@ -1486,7 +1486,7 @@ def to_web(self) -> Any:
 
             agent = Agent('openai:gpt-5')
 
-            @agent.tool
+            @agent.tool_plain
             def get_weather(city: str) -> str:
                 return f'The weather in {city} is sunny'