Code typo in docs (easy fix)

In section "Adding Specs to a Model" of the "Getting Started" page, the `processed_image` field should be defined with `ProcessedImageField` instead of `ImageSpecField`

``` python
from django.db import models
from imagekit.models.fields import ProcessedImageField

class Photo(models.Model):
    processed_image = ImageSpecField(format='JPEG', options={'quality': 90})
```
