Update Overview.bs

<a>block container boxes</a> that contains only <a>block-level</a> boxes.
->
<a>block container boxes</a> that contain only <a>block-level</a> boxes.

Author: ewilligers

css-break-3/Overview.bs

@@ -418,7 +418,7 @@ Breaks Between Lines: 'orphans', 'widows'</h3>
 
   <p>
     The 'widows' and 'orphans' properties have no effect
-    on <a>block container boxes</a> that contains only <a>block-level</a> boxes.
+    on <a>block container boxes</a> that contain only <a>block-level</a> boxes.
   </p>
   <p>
     The 'orphans' property specifies the minimum number