fix using use_attestation_challenge error parameter code for missing attestation

Author: Micha Kraus

draft-ietf-oauth-attestation-based-client-auth.md

@@ -325,7 +325,7 @@ To validate an HTTP request which contains the client attestation headers, the r
 2. There is precisely one OAuth-Client-Attestation-PoP HTTP request header field, where its value is a single well-formed JWT conforming to the syntax outlined in [](#client-attestation-pop-jwt).
 3. The signature of the Client Attestation PoP JWT obtained from the OAuth-Client-Attestation-PoP HTTP header verifies with the Client Instance Key contained in the `cnf` claim of the Client Attestation JWT obtained from the OAuth-Client-Attestation HTTP header.
 
-An error parameter according to Section 3 of {{RFC6750}} SHOULD be included to indicate why a request was declined. If the Client Attestation is absent or not using an expected server-provided challenge, the value `use_attestation_challenge` can be used to indicate that an attestation with a server-provided challenge was expected. If the attestation and proof of possession was present but could not be successfully verified, the value `invalid_client_attestation` is used.
+An error parameter according to Section 3 of {{RFC6750}} SHOULD be included to indicate why a request was declined. If the Client Attestation is not using an expected server-provided challenge, the value `use_attestation_challenge` can be used to indicate that an attestation with a server-provided challenge was expected. If the attestation and proof of possession was present but could not be successfully verified, the value `invalid_client_attestation` is used.
 
 ## Client Attestation at the Token Endpoint {#token-endpoint}