Chapter 4.5 freq_words missing nltk library call

The final `freq_words` function  is missing `nltk` library call before `FreqDist()`

**Before:**

```python
def freq_words(file, min=1, num=10, verbose=False):
    freqdist = FreqDist()
...
```

**After:**

```python
def freq_words(file, min=1, num=10, verbose=False):
    freqdist = nltk.FreqDist()
...
```