[css-text-decor-4] Fix typo

Author: fantasai

css-text-decor-4/Overview.bs

@@ -112,7 +112,7 @@ Text Decoration Lines: the 'text-decoration-line' property</h3>
 Text Decoration Line Thickness: the 'text-decoration-thickness' property</h3>
 
 	<pre class="propdef">
-	Name: text-decoration-thickenss
+	Name: text-decoration-thickness
 	Value: auto | from-font | <<length>>
 	Initial: auto
 	Applies to: all elements