tokenize function code in data_utils.py is incorrect

with the test intention that
```
>>> tokenize('Bob dropped the apple. Where is the apple?')
    ['Bob', 'dropped', 'the', 'apple', '.', 'Where', 'is', 'the', 'apple', '?']
```

we should write like this:
```
def tokenize(sent):
    return [x for x in re.findall(r"\w+(?:'\w+)?|[^\w\s]", sent)]
```