[css-grid] fix example markup

Author: tabatkins

css-grid/Overview.bs

@@ -1572,7 +1572,8 @@ Resolved Values</h4>
 	the resolved value is simply the computed value.
 
 	<div class="example">
-		<pre>
+		<pre class="lang-html">
+			&lt;style>
 			#grid {
 				width: 500px;
 				grid-template-columns:
@@ -1590,6 +1591,7 @@ Resolved Values</h4>
 			  var gridElement = document.getElementById("grid");
 			  getComputedStyle(gridElement).gridTemplateColumns;
 			  // [a] 50px [b] 320px [b c d] repeat(2, [e] 40px) repeat(4, 0px) 50px
+			&lt;/script>
 		</pre>
 	</div>