Fix repo instructions border radius

What does this MR do and why?

Fix border radius of empty repo code blocks (top is 0, bottom is 4px)

• Removes .code class so it matches other areas (like the MR checkout instructions code blocks)

What does this MR not do?

Fix the border radius in the This repository is empty... card (fixed as part of Remove gl-new-card styles (!162204 - merged))

Screenshots or screen recordings

Before	After

How to set up and validate locally

Create an empty project

Links

• ✂️ UX Paper Cuts 17.3 → Dark mode - Design Support (#452231 - closed)