🐛 `map` not working
I didn't assign the output 🤦‍♂️

```python
dataset.map(test)
```

should be :

```python
dataset = dataset.map(test)
```
I'm trying to run a basic example (mapping function to add a prefix).  
[Here is the colab notebook I'm using.](https://colab.research.google.com/drive/1YH4JCAy0R1MMSc-k_Vlik_s1LEzP_t1h?usp=sharing)

```python
import nlp

dataset = nlp.load_dataset('squad', split='validation[:10%]')

def test(sample):
    sample['title'] = "test prefix @@@ " + sample["title"]
    return sample

print(dataset[0]['title'])
dataset.map(test)
print(dataset[0]['title'])
```
Output :
> Super_Bowl_50
Super_Bowl_50

Expected output :
> Super_Bowl_50
test prefix @@@ Super_Bowl_50