Response Endpoint may not be suffciently described

[paulbastian]

as discussed on DCP call

[Sakurann]

to add color, OID4VP currently uses a term Response Endpoint to point to a Verifier's endpoint that receives the response from the Wallet with requested VC(s). but that might not be stated clearly enough in the text

[Sakurann]

Agreed to replace Response Endpoint with Response URI. (to be consistent with response_uri parameter name).

Also Update definition to response_uri to say it is a URL.