[css-break-3] Wording fix.

Author: heycam

css-break-3/Overview.bs

@@ -731,7 +731,7 @@ Unforced Breaks</h3>
   <p>
     If even that does not lead to sufficient break points,
     <a href="#valdef-box-decoration-break-clone">cloned margins/border/padding</a>
-    at on the block-end side are truncated;
+    at the block-end side are truncated;
     and if more room is still needed,
     <a href="#valdef-box-decoration-break-clone">cloned margins/border/padding</a>
     are truncated at the block-end side as well.