gpui: Fix incorrect colors comment (#38929)

| Before | After |
| - | - |
| <img width="466" height="207" alt="SCR-20250926-khst"
src="https://github.com/user-attachments/assets/c28a9ea8-3d22-458c-a683-b2fabe275a04"
/> | <img width="480" height="215" alt="SCR-20250926-kgru"
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/> |

Release Notes:

- N/A

crates/gpui/src/color.rs

@@ -362,7 +362,7 @@ pub const fn transparent_black() -> Hsla {
     }
 }
 
-/// Transparent black in [`Hsla`]
+/// Transparent white in [`Hsla`]
 pub const fn transparent_white() -> Hsla {
     Hsla {
         h: 0.,