ClassLabel docs: Correct value for unknown labels

[l-uuz]

This small change fixes the documentation to to be compliant with what happens in encode_example.

datasets/src/datasets/features/features.py

Lines 1126 to 1129 in e71b0b1

	# Allowing -1 to mean no label.
	if not -1 <= example_data < self.num_classes:
	raise ValueError(f"Class label {example_data:d} greater than configured num_classes {self.num_classes}")
	return example_data