Use curly brackets to fix client_spec ruby 3 matcher

What does this MR do and why?

This fixes the client_spec failures currently happening in Ruby 3 only (https://gitlab.com/gitlab-org/gitlab/-/jobs/3477546463). See this MR for a similar issue and more context: !105238 (merged)

ruby 3 pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/723323089

ruby 2 pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/723324923

MR acceptance checklist

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