Improve code example in marbles.core.TestCase docstring

### Description
The `put` function in the code example provided in [`marbles.core.TestCase`](https://marbles.readthedocs.io/en/stable/core.html#marbles.core.TestCase) is indented strangely.

```python
import requests
import marbles.core

def put(cls, endpoint, data=None):
return Response(status_code=409,
reason=('The request could not be completed '
'due to a conflict with the current '
'state of the target resource.'))

class ResponseTestCase(marbles.core.TestCase):

def test_create_resource(self):
endpoint = 'http://example.com/api/v1/resource'
data = {'id': 1, 'name': 'Little Bobby Tables'}

res = requests.put(endpoint, data=data)
self.assertEqual(
res.status_code,
201,
note=res.reason
```

### Intended audience
Users

### Additional context
A better-motivated example would be appropriate here:
1. tests shouldn't be hitting external services
1. this example doesn't demonstrate the advantage of capturing more information about the test author's intent