[css-shapes-1][editorial] fix stray number-percentages

Author: svgeesus

css-shapes-1/Overview.bs

@@ -381,7 +381,7 @@ Supported Shapes</h3>
 			via offsets from the top and left edge of the [=reference box=],
 			and a specified width and height.
 
-			The four <<length-percentages>> define,
+			The four <<length-percentage>>s define,
 			respectively,
 			the inset from the left edge of the [=reference box=],
 			the inset from the top edge of the [=reference box=],