[css-grid-1] Fix example. Fixes #4991

Author: fantasai

css-grid-1/Overview.bs

@@ -776,7 +776,7 @@ Reordering and Accessibility</h2>
 		This layout can be easily achieved with grid layout:
 
 		<pre class="lang-css">
-			main { display: grid;
+			body { display: grid;
 			       grid: "h h h"
 			             "a b c"
 			             "f f f";