[css-color-5] Clarified that sole item in color-mix() is returned in the specified color space. Fix w3c#12234

Author: svgeesus

css-color-5/Overview.bs

@@ -279,7 +279,8 @@ Mixing Colors: the ''color-mix()'' Function {#color-mix}
 	3. Let |alpha mult| be <code>1 - |leftover|</code>,
 		interpreting |leftover| as a number between 0 and 1.
 	4. If |items| is length 1,
-		set |color| to the color of that sole item.
+		set |color| to the color of that sole item,
+		converted to the specified interpolation <<color-space>>.
 
 		Otherwise: