nit: Fix invalid html within example (#11758)

Author: tonyraoul

css-grid-3/Overview.bs

@@ -295,7 +295,7 @@ Reordering and Accessibility</h3>
 			  <div class="item tall">3</div>
 			  <div class="item wide">4</div>
 			  <div class=item>5</div>
-			  <div class=item>6/div>
+			  <div class=item>6</div>
 			  <div class=item>7</div>
 			</section>
 			<style>