[css-scrollbar] Fix markup

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -270,11 +270,15 @@ Value Definitions</h3>
 			authors should use ''overflow: hidden'' instead of ''scrollbar-width: none''.
 	</dl>
 
-	Note: Users who find the ''thin'' style of scrollbars unusable
-	can include the following rule in their [=user style sheet=]:
-	<pre><code class=lang-css>* { scrollbar-width: auto !important; }</code></pre>
-	This will ensure that all scrollbars are sized as per OS and UA settings
-	regardless of author styles.
+	<div class=note>
+		Note: Users who find the ''thin'' style of scrollbars unusable
+		can include the following rule in their [=user style sheet=]:
+
+		<pre><code class=lang-css>* { scrollbar-width: auto !important; }</code></pre>
+
+		This will ensure that all scrollbars are sized as per OS and UA settings
+		regardless of author styles.
+	</div>
 
 	UAs must apply the 'scrollbar-width' value set on the root element to the viewport.