Fix luke docstring
Author: qqaatw
Pull request: yes

# What does this PR do?

The output logits of `LukeForEntitySpanClassification` is in `(batch_size, entity_length, config.num_labels)` shape instead of `(batch_size, config.num_labels)`, which is that of `LukeForEntityClassification`.

https://github.com/huggingface/transformers/blob/8b67f20935e48b26c5803cf31e0e89b9cfaa22ab/tests/models/luke/test_modeling_luke.py#L402-L404

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Fixes # (issue)


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