Fix Bundler element in matrix array
What does this MR do and why?
This fixes a syntax error from !667 (diffs).
This is the image we're looking for: https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/4068051454
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Rémy Coutable