Add reference to RFC7591 for token_endpoint_auth_method (#440)

2 approvals. open for more than a week. not normative

openid-4-verifiable-credential-issuance-1_0.md

@@ -630,7 +630,7 @@ The following are the extension parameters to the Token Request used in the Pre-
 * `pre-authorized_code`: The code representing the authorization to obtain Credentials of a certain type. This parameter MUST be present if the `grant_type` is `urn:ietf:params:oauth:grant-type:pre-authorized_code`.
 * `tx_code`: OPTIONAL. String value containing a Transaction Code value itself. This value MUST be present if a `tx_code` object was present in the Credential Offer (including if the object was empty). This parameter MUST only be used if the `grant_type` is `urn:ietf:params:oauth:grant-type:pre-authorized_code`.
 
-Requirements around how the Wallet identifies and, if applicable, authenticates itself with the Authorization Server in the Token Request depends on the Client type defined in Section 2.1 of [@!RFC6749] and the Client authentication method indicated in the `token_endpoint_auth_method` Client metadata. The requirements specified in Sections 4.1.3 and 3.2.1 of [@!RFC6749] MUST be followed.
+Requirements around how the Wallet identifies and, if applicable, authenticates itself with the Authorization Server in the Token Request depends on the Client type defined in Section 2.1 of [@!RFC6749] and the Client authentication method indicated in the `token_endpoint_auth_method` Client metadata (as defined in [@!RFC7591]). The requirements specified in Sections 4.1.3 and 3.2.1 of [@!RFC6749] MUST be followed.
 
 For the Pre-Authorized Code Grant Type, authentication of the Client is OPTIONAL, as described in Section 3.2.1 of OAuth 2.0 [@!RFC6749], and, consequently, the `client_id` parameter is only needed when a form of Client Authentication that relies on this parameter is used.