[css-box-4] Update Changes list

Author: fantasai

css-box-4/Overview.bs

@@ -544,11 +544,21 @@ Borders {#borders}
 	Both sets of properties control the same set of borders:
 	they are just different ways of indexing each side.
 
+Changes Since CSS Level 3 {#changes}
+=========================
+
+	The following changes have been made to this module
+	since <a href="https://www.w3.org/TR/css-box-3/">Level 3</a>:
+	<ul>
+		<li>Adding the 'margin-trim' property.
+		<li>Adapting the prose slightly to account for vertical <a>writing modes</a>.
+	</ul>
+
 Changes Since CSS Level 2 {#changes}
 =========================
 
 	The following changes have been made to this module
-	since <a href="https://www.w3.org/TR/CSS2/box.html">CSS Level 2</a>:
+	since <a href="https://www.w3.org/TR/CSS2/box.html">Level 2</a>:
 	<ul>
 		<li>Adding the 'margin-trim' property.
 		<li>Adapting the prose slightly to account for vertical <a>writing modes</a>.