clarify that nonce in key attestation is c_nonce

[Sakurann]

in this line https://openid.github.io/OpenID4VCI/openid-4-verifiable-credential-issuance-wg-draft.html#appendix-D.1-4.1.2.7

[Sakurann]

link in the issue is broken. new one is https://openid.github.io/OpenID4VCI/openid-4-verifiable-credential-issuance-1_1-wg-draft.html#appendix-D.1-4.1.2.7