doc: input_signature cannot be a numpy array unlike documentation says

[JohannesBuchner]

Describe the bug
The readme of from_keras (here: https://github.com/onnx/tensorflow-onnx?tab=readme-ov-file#from_keras-tf-20-and-newer)
says: "input_signature: a tf.TensorSpec or a numpy array defining the shape/dtype of the input"

However, this is wrong in two levels:
First, providing a single value gives an error saying that a list or tuple of TensorSpec was expected.
Second, providing a tuple with numpy arrays is also not allowed.

Urgency

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 18.04*): Ubuntu 24.04
• TensorFlow Version: 2.19.0
• Python version: 3.12
• ONNX version (if applicable, e.g. 1.11*): 1.17.0
• ONNXRuntime version (if applicable, e.g. 1.11*): 1.19.2

To Reproduce

Screenshots

Additional context