doc: Invalid index of keystone highlight in Rocky

[Brin Zhang]

In keystone highlight [1], the index [2] is invalid.

[1] https://releases.openstack.org/rocky/highlights.html#keystone-identity-service
[2] https://github.com/openstack/releases/blob/master/deliverables/rocky/keystone.yaml#L59

[Brin Zhang (zhangbailin)]

This is the effective address: https://docs.openstack.org/keystone/latest/admin/case-insensitive.html

[Colleen Murphy (krinkle)]

Please do not report trivial errors like this, just go ahead and submit the correction.

In this case the error was not in keystone but was in the releases project, so marking this as invalid.