[Docs] map() example uses undefined `tokenizer` — causes NameError

## Description

The current documentation example for `datasets.Dataset.map()` demonstrates batched processing but uses a `tokenizer` object without defining or importing it. This causes an error every time it's copied.

Here is the problematic line:

```python
# process a batch of examples
>>> ds = ds.map(lambda example: tokenizer(example["text"]), batched=True)
```
This assumes the user has already set up a tokenizer, which contradicts the goal of having self-contained, copy-paste-friendly examples.

## Problem
Users who copy and run the example as-is will encounter:

```python
NameError: name 'tokenizer' is not defined
```

This breaks the flow for users and violates HuggingFace's documentation principle that examples should "work as expected" when copied directly.

## Proposal
Update the example to include the required tokenizer setup using the Transformers library, like so:

```python
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("bert-base-uncased")
ds_tokenized = ds.map(lambda example: tokenizer(example["text"]), batched=True)
```
This will help new users understand the workflow and apply the method correctly.

## Note
This PR complements ongoing improvements like #7700, which clarifies multiprocessing in .map(). My change focuses on undefined tokenizer — causes NameError