Fix typo on tests style guide
What does this MR do?
Fix typo introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26825
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26825#note_156435731
Does this MR meet the acceptance criteria?
- [-] Changelog entry added, if necessary
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Documentation created/updated via this MR -
Documentation reviewed by technical writer or follow-up review issue created - [-] Tests added for this feature/bug
- [-] Tested in all supported browsers
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Conforms to the code review guidelines - [-] Conforms to the merge request performance guidelines
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Conforms to the style guides - [-] Conforms to the database guides
- [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
- [-] Security reports checked/validated by reviewer