[css3-background] fix markup error

Author: dauwhe

css-backgrounds/Overview.src.html

@@ -2832,7 +2832,7 @@ <h3 id="the-box-shadow">Drop Shadows: the 'box-shadow' property</h3>
   <p>The components of each <var>&lt;shadow&gt;</var> are interpreted as follows:
 
   <dl>
-    <dt><dfn id="shadow-offset-x" title=''>1st <var>&lt;length&gt;</dfn></var></dfn>
+    <dt><dfn id="shadow-offset-x" title=''>1st <var>&lt;length&gt;</var></dfn>
     <dd>
       Specifies the <dfn>horizontal offset</dfn> of the shadow.
       A positive value draws a shadow that is offset to the right of the box,