Fix formatting function to ensure correct fine-tuning in gemma-peft.md

[dboyker]

Thanks for the nice tutorial on gemma + peft !!

After following it, the script throws this warning:

You passed a dataset that is already processed (contains an input_ids field) together with a formatting function. Therefore formatting_func will be ignored. Either remove the formatting_func or pass a dataset that is not already processed.

As formatting_func is ignored, the model is not correctly fine-tuned. The script works but there is no guarantee that the model ouput the format Quote: [...] Author: [...].

The line 115 is the one which prevents this:
data = data.map(lambda samples: tokenizer(samples["quote"]), batched=True). Executing it adds the input_ids column to the dataset which, together with the formatting_function arg, then trigger the warning as seen here: https://github.com/huggingface/trl/blob/main/trl/trainer/sft_trainer.py#L938-L944

Removing the line 115 is safe regarding the tokenization. Indeed, the tokenizer is infered in the SFTTrainer __init__: https://github.com/huggingface/trl/blob/main/trl/trainer/sft_trainer.py#L639-L650

In addition, formatting_func is modified in this PR to avoid raising the following error: AttributeError: 'list' object has no attribute 'endswith'. It now returns a string instead of a list (+ it does not slice the quote and author).