[css-mixins-1[editorial]] Correct syntax in example, fix #12813

Author: svgeesus

css-mixins-1/Overview.bs

@@ -834,8 +834,8 @@ making them reusable and customizable with arguments.
 
 	<pre highlight=css>
 		@mixin --gradient-text(
-			--from type(color): mediumvioletred,
-			--to type(color): teal,
+			--from type(<<color>>): mediumvioletred,
+			--to type(<<color>>): teal,
 			--angle: to bottom right,
 		) {
 			color: env(--from, env(--to));