[css2][editorial] minor markup fix

Author: tabatkins

css2/Overview.bs

@@ -9299,8 +9299,8 @@ dimensions must satisfy this constraint:</p>
 
   <blockquote>
     <p>'top' + 'margin-top' + 'border-top-width' + 'padding-top' +      'height'
-\+ 'padding-bottom' + 'border-bottom-width' +      'margin-bottom' + 'bottom'
-= height of containing block </p>
+      + 'padding-bottom' + 'border-bottom-width' +      'margin-bottom' + 'bottom'
+      = height of containing block </p>
   </blockquote>
 
 <p>If all three of 'top', 'height', and 'bottom' are auto, set 'top'