[css-syntax] Markup fix.

Author: tabatkins

css-syntax/Overview.bs

@@ -314,7 +314,7 @@ The input byte stream</h3>
 			for the sequence of <code>XX</code> bytes,
 			interpreted as <code>ASCII</code>.
 
-			<details class='why'>
+			<details class='note'>
 				<summary>What does that byte sequence mean?</summary>
 
 				The byte sequence above,
@@ -328,7 +328,7 @@ The input byte stream</h3>
 			if it was anything else except failure,
 			use the return value as the fallback encoding.
 
-			<details class='why'>
+			<details class='note'>
 				<summary>Why use utf-8 when the declaration says utf-16?</summary>
 
 				The bytes of the encoding declaration spell out “<code>@charset "…";</code>” in ASCII,