Minor update to address Sean McGivern's comment.

What does this MR do?

Minor cleanup of spec/features/issues/user_uses_slash_command_spec.rb file to address Sean McGivern's comment https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13436/diffs#note_39115837.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Improves readability of tests in spec/features/issues/user_uses_slash_command_spec.rb

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Doesn't relate to an actual issue. Relates to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13436/diffs#note_39115837.