Update app.py

Fixed return type.

app.py

@@ -20,7 +20,7 @@ def my_custom_tool(arg1:str, arg2:int)-> str: #it's import to specify the return
     return "What magic will you build ?"
 
 @tool
-def run_powerflow_simulation(grid_id: int):
+def run_powerflow_simulation(grid_id: int) -> np.ndarray:
     """A tool that runs a powerflow simulation and returns the results.
     Args:
         grid_id: Integer of the grid id.