improve wording where we default to transparent

Author: RobinMalfait

src/docs/scrollbar-color.mdx

@@ -102,7 +102,7 @@ Use utilities like `scrollbar-thumb-sky-700` and `scrollbar-track-sky-100` to co
 
 </Figure>
 
-You can set the thumb and track colors independently. If one side is not set, it defaults to transparent.
+You can set the thumb and track colors independently. If you set one without the other, the unset color defaults to transparent.
 
 ### Changing the opacity