examples: only use keep_linebreaks when reading TXT files
Hi,

this is a follow-up (bug-fix) PR for #13150.

It turns out - as reported in #13312 - that the `keep_linebreaks` argument only works, when Datasets extension is `text`.

I used this logic to only pass the `keep_linebreaks` argument, when extension is `text`, simplified as:

```
dataset_args = {}

if extension == "text":
    dataset_args["keep_linebreaks"] = True

dataset = load_dataset(extension, data_files=data_files, **dataset_args)

print(dataset["train"][0])
```

When `keep_linebreaks` was set to `True` and reading in a text file, the output looks like:

```bash
{'text': 'Heute ist ein schöner Tach\n'}
```

For `keep_linebreaks` set to `False` the output looks like:

```bash
{'text': 'Heute ist ein schöner Tach'}
```

So the proposed way is working with the `dataset_args` argument. I also checked all examples that they're working when passing a CSV dataset.
