gpui: Rename rounded_md to rounded_sm (#26228)

This PR renames the `rounded_md` style method to `rounded_sm`.

Follow up to https://github.com/zed-industries/zed/pull/26221, which
freed up the `rounded_sm` name.

Release Notes:

- N/A

crates/repl/src/notebook/cell.rs

@@ -646,7 +646,7 @@ impl Render for CodeCell {
                                             // .w_full()
                                             // .mt_3()
                                             // .p_3()
-                                            // .rounded_md()
+                                            // .rounded_sm()
                                             // .bg(cx.theme().colors().editor_background)
                                             // .border(px(1.))
                                             // .border_color(cx.theme().colors().border)