Fix slash commands dropdown description

What does this MR do?

It fixes slash commands dropdown description mis-alignment on Firefox

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

Why was this MR needed?

Fix slash commands dropdown description on Firefox is mis-aligned, as pointed out in the following issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/40190

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
• Internationalization required/considered

What are the relevant issue numbers?

#40190 (moved)