docs: add VAULT_OIDC_AUD to kicker

kicker.json

@@ -246,6 +246,11 @@
           "description": "The Vault server base API url",
           "mandatory": true
         },
+        {
+          "name": "VAULT_OIDC_AUD",
+          "description": "The `aud` claim for the JWT",
+          "default": "$CI_SERVER_URL"
+        },
         {
           "name": "VAULT_ROLE_ID",
           "description": "The [AppRole](https://www.vaultproject.io/docs/auth/approle) RoleID",