[css-line-grid] remove duplicate word

Author: dauwhe

css-line-grid/Overview.bs

@@ -478,7 +478,7 @@ Snapping Block Boxes: the 'box-snap' property</h3>
 	<dt><dfn>block-end</dfn>
 		<dd>The block-end edge is snapped to the nearest grid line.
 	<dt><dfn>center</dfn>
-		<dd>The block is centered centered between
+		<dd>The block is centered between
 		one of the baseline grid's text-over baselines and a subsequent
 		(but not necessarily consecutive) text-under baseline.
 	<dt><dfn>baseline</dfn>