Update kerberos docs for the new multi-backend format (#261)

# Description

This was changed in #257, but we forgot to update the docs then.

Author: nightkr

docs/modules/secret-operator/pages/secretclass.adoc

@@ -78,7 +78,9 @@ spec:
     kerberosKeytab:
       realmName: CLUSTER.LOCAL
       kdc: krb5-kdc
-      adminServer: krb5-kdc
+      admin:
+        mit:
+          kadminServer: krb5-kdc
       adminKeytabSecret:
         namespace: default
         name: secret-provisioner-keytab
@@ -88,7 +90,7 @@ spec:
 `kerberosKeytab`:: Declares that the `kerberosKeytab` backend is used.
 `kerberosKeytab.realmName`:: The name of the Kerberos realm. This should be provided by the Kerberos administrator.
 `kerberosKeytab.kdc`:: The hostname of the Kerberos Key Distribution Center (KDC). This should be provided by the Kerberos administrator.
-`kerberosKeytab.adminServer`:: The hostname of the Kerberos Admin Server. This should be provided by the Kerberos administrator.
+`kerberosKeytab.admin.mit.kadminServer`:: The hostname of the Kerberos Admin Server. This should be provided by the Kerberos administrator.
 `kerberosKeytab.adminKeytabSecret`:: Reference (`name` and `namespace`) to a K8s `Secret` object where a keytab with administrative privileges is stored in the key `keytab`.
 `kerberosKeytab.adminPrincipal`:: The name of the Kerberos principal to be used by the Secret Operator. This should be provided by the Kerberos administrator. The credentials for this principal must be stored in the keytab (`adminKeytabSecret`).