[css-grid] Fix copypaste error. Fixes #1356.

Author: tabatkins

css-grid/Overview.bs

@@ -2082,7 +2082,7 @@ Implicit Track Sizing: the 'grid-auto-rows' and 'grid-auto-columns' properties</
 	to find the size of the implicit tracks.
 	The first <a>implicit grid track</a> before the <a>explicit grid</a>
 	receives the first specified size, and so on forwards;
-	and the last <a>implicit grid track</a> before the <a>explicit grid</a>
+	and the last <a>implicit grid track</a> after the <a>explicit grid</a>
 	receives the last specified size, and so on backwards.
 
 	<div class="example">