Update `url_prefix` in Custom Domain Example

Under the [Custom Domain Name Example](https://aws.github.io/chalice/topics/configfile.html#custom-domain-name), we have the following:
```
"url_prefixes": ["foo", "bar"],
```
I think it should be:

```
"url_prefix": "foo",
```

Open the Custom Domain Name Example at the linked configfile.html page and compare the documented configuration key with the issue's correction. Done means the example uses the singular url_prefix form and the rendered documentation shows the corrected snippet.

Written by the indexing model from the issue text.