[Inference Client] fix param docstring and deprecate labels param in zero-shot classification tasks

[hanouticelina]

This PR does two things:

• fixes a bug in the parameter docstring formatting when it's not an optional one, see this comment for more context.
• deprecates labels parameter in InferenceClient.zero_shot_classification and InferenceClient.zero_shot_image_classification in favor of candidate_labels.

This PR should be merged before #2664.

[HuggingFaceDocBuilderDev]

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[Wauplin]

Thanks!