Include Cache-Control and Pragma headers in token exchange response example

According to https://www.rfc-editor.org/rfc/rfc6749#section-5.1, token exchange responses MUST contain the following headers

```
Cache-Control: no-store
Pragma: no-cache
```

Consider adding these to https://indieauth.spec.indieweb.org/#example-12 to make it more likely that people implementing IndieAuth servers based purely on the IndieAuth spec include them

Start with IndieAuth specification Example 12 and compare its token exchange response with RFC 6749 section 5.1. Done means the example includes Cache-Control: no-store and Pragma: no-cache headers.

Written by the indexing model from the issue text.