Fix typo in T5Block error message

[Mingosnake]

What does this PR do?

Fixes typo in modeling_t5.py
line 680: f"{'2 (past / key) for cross attention. ' if expected_num_past_key_values == 4 else ''}"
->f"{'2 (key / value) for cross attention. ' if expected_num_past_key_values == 4 else ''}"
in class T5Block

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[ArthurZucker]

Thanks you for the nit

[HuggingFaceDocBuilderDev]

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