Improve the positional args error message for dspy.Predict

[chenmoneygithub]

To make that even more actionable. A sample code and error:

dspy.Predict("question, context -> answer")("What is the capital of France?")

error:

ValueError: Positional arguments are not allowed when calling `dspy.Predict`, must use keyword arguments that match your signature input fields: 'question, context'. For example: `predict(question=input_value, ...)`.