Name change in the repo broke the links to the OpenAPI pages

[NSeydoux]

According the commit comment of https://github.com/w3c-ccg/vc-api/commit/4cd94ab7c85c8cfdace7e3dc849bd9f31a3232b3, it intended to fix links after the API rename, however the links to the OpenAPI GH pages from the Readme still use the former repository name, leading to a 404. I would have expected GH to redirect the GH pages after the repository rename, but it appears that it is not the case. Anything that reads https://w3c-ccg.github.io/vc-http-api/ should be changed to https://w3c-ccg.github.io/vc-api/.