Update util.py

Changed string to a literal

Author: JPPhoto

invokeai/app/invocations/util.py

@@ -11,4 +11,4 @@ def validate_weights(weights: Union[float, list[float]]) -> None:
 def validate_begin_end_step(begin_step_percent: float, end_step_percent: float) -> None:
     """Validate that begin_step_percent is less than or equal to end_step_percent"""
     if begin_step_percent > end_step_percent:
-        raise ValueError(f"Begin step percent must be less than or equal to end step percent")
+        raise ValueError("Begin step percent must be less than or equal to end step percent")