Use `response_mode=cookie` in example for "5.2 Unauthenticated Requests"

Hi @jaredhanson

This is about https://datatracker.ietf.org/doc/draft-hanson-oauth-cookie-response-mode/ (which apparently is a more recent version of the document in this repo).

In section "5.2. Unauthenticated Requests" you give an example how a protected resource may respond to an unauthenticated request by
* setting the `WWW-Authenticate` response header field with Thomas Broyer's `Cookie` auth scheme and
* setting an HTML login form as the response body content.

To me it would seem more appropriate to give an example where the response contains a redirect to the authorization endpoint using the `response_mode=cookie` that this spec defines.

See also https://github.com/aaronpk/draft-hanson-oauth-cookie-response-mode/issues/2

Locate section 5.2, “Unauthenticated Requests,” in the draft source and review the current example. Compare it with the newer IETF document and the linked related issue, then update the example to reflect the requested cookie response mode and verify the document still builds.

Written by the indexing model from the issue text.