Fix Documentation: Event Handler REST API CORS example

In the example for [CORS](https://awslabs.github.io/aws-lambda-powertools-python/develop/core/event_handler/api_gateway/#cors), line 3 should read:

```python
from aws_lambda_powertools.event_handler.api_gateway import APIGatewayRestResolver, CORSConfig
```

Or `CORSConfig` should be added to `__init__.py`.