Fix Rubocop offenses in Elastic cluster reindexing service spec

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183826



Merged-by: Arturo Herrero <arturo.herrero@gmail.com>
Approved-by: Lukas Wanko <lwanko@gitlab.com>
Approved-by: Smriti Garg <sgarg@gitlab.com>
Reviewed-by: Lukas Wanko <lwanko@gitlab.com>