Update to example since it is missing the nameid_format

`nameid_format: "urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"`

When setting `attributes.principal: "nameid:persistent` the `nameid_format` option should be added as per [our documentation here](https://www.elastic.co/guide/en/elasticsearch/reference/current/saml-guide-authentication.html#saml-attribute-mapping-nameid)

Author: Robbert Hink

x-pack/docs/en/security/authentication/saml-guide.asciidoc

@@ -301,6 +301,7 @@ xpack.security.authc.realms.saml.saml1:
   sp.acs: "https://kibana.example.com/api/security/saml/callback"
   attributes.principal: "nameid:persistent"
   attributes.groups: "roles"
+  nameid_format: "urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"
 ------------------------------------------------------------
 
 [[saml-user-properties]]