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/editor/src/element.rs

@@ -2717,7 +2717,7 @@ impl EditorElement {
                     .flex_basis(Length::Definite(DefiniteLength::Fraction(0.667)))
                     .pl_0p5()
                     .pr_5()
-                    .rounded_md()
+                    .rounded_sm()
                     .shadow_md()
                     .border_1()
                     .map(|div| {