In group_texts function, drop last block if smaller than block_size
Author: billray0259
Pull request: yes

# What does this PR do?

Adds one line to both the English and Spanish versions of the [Language modeling task documentation](https://huggingface.co/docs/transformers/tasks/language_modeling) in the `group_texts` function which drops the last block if the block is smaller than `block_size`. The absence of this line causes this exception to be thrown later when training the model:

`ValueError: Unable to create tensor, you should probably activate truncation and/or padding with 'padding=True' 'truncation=True' to have batched tensors with the same length.`

This line is present in the related documentation, [Fine-tuning a masked language model](https://huggingface.co/course/chapter7/3?fw=pt).

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Fixes [# 17882](https://github.com/huggingface/transformers/issues/17882)


## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

@SaulLu, @sgugger

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Comments:
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