I checked the Editors' Draft version. The example in 4.1.2 uses a parameter called credentials.
I believe that using credential_configuration_ids is appropriate for the current latest specification.
Current example:
(Percent encoding is decoded for readability.)
GET /credential_offer?credential_offer={"credential_issuer":"https://credential-issuer.example.com","credentials":["UniversityDegree_JWT","org.iso.18013.5.1.mDL"],"grants":{"urn:ietf:params:oauth:grant-type:pre-authorized_code":{"pre-authorized_code":"oaKazRN8I0IbtZ0C7JuMn5","tx_code":{}}}}