Minor readability improvement in dataloader.ipynb (#461)

* Minor readability improvement in dataloader.ipynb

- The tokenizer and encoded_text variables at the root level are unused.
- The default params for create_dataloader_v1 are confusing, especially for the default batch_size 4, which happens to be the same as the max_length.

* readability improvements

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Co-authored-by: rasbt <mail@sebastianraschka.com>