fix: add missing private arg to proving.credential() (#925)

src/keri/vdr/credentialing.py

@@ -805,6 +805,7 @@ def create(self, regname, recp: str, schema, source, rules, data, private: bool
                                     recipient=recp,
                                     data=data,
                                     source=source,
+                                    private=private,
                                     private_credential_nonce=private_credential_nonce,
                                     private_subject_nonce=private_subject_nonce,
                                     rules=rules,