Update the travel advice index example to support new style links
Update the travel advice index example to support new style links

As per https://github.com/alphagov/frontend/pull/1643 we allow the frontend to support both the old style and new style of `details` multi-level links.

I've done this for only one child travel advice to ensure that it copes with both kinds of formats, once that's all deployed, we can change all of them.
alphagov/govuk-content-schemas,alphagov/govuk-content-schemas