[css-values-5][editorial] fix markup

Author: tabatkins

css-values-5/Overview.bs

@@ -1128,7 +1128,7 @@ Defining the Interpolation Map</h4>
 			it is treated the same as two stops with the same <<output-value>>.
 
 			<pre class=prod>
-				<input-position> = <percentage> | <number> | <dimension>
+				<<input-position>> = <<percentage>> | <<number>> | <<dimension>>
 			</pre>
 
 			Note: <<output-value>> is not given a grammar here,