[Atlas] Update Okta LDAP authorization example

Description

Since the SERVER-43233 has been backported, the step 5 in the Configure Atlas for LDAP Authorization needs to be updated on this page: https://docs.atlas.mongodb.com/security-ldaps-okta/index.html#configure-service-for-ldap-authorization

Contents before the change:

ou=groups,dc=<okta-instance-id>,dc=okta,dc=com??sub?(&(objectClass=groupofUniqueNames)(uniqueMember={USER}))

Contents after the change:

ou=groups,dc=<okta-instance-id>,dc=okta,dc=com?dn?sub?(&(objectClass=groupofUniqueNames)(uniqueMember={USER}))

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)