Incorrect documentation for sync

In [README.rst](https://github.com/contentful/contentful.py/blob/master/README.rst), the `Synchronization` section has this code sample:

```
sync = sync.next(client) # equivalent to client.sync(sync_token=sync.next_sync_token)
```

The example code given in the comment doesn't work. It needs to be:

```
client.sync({'sync_token': sync.next_sync_token})
```