change examples in the main body to mdoc credential format.

[Kristina Yasuda]

to make clear that the specification supports multiple credential formats.

based on the conversation in openid / connect / issues / #1915 - "Verifiable Credentials" causes people to believe OID4VC supports W3C VCs only — Bitbucket

[Mark Haine]

Migrated to GitHub

https://github.com/openid/OpenID4VP