[css-grid] Fix error in example selector

Author: fantasai

css-grid/Overview.bs

@@ -614,9 +614,9 @@ Reordering and Accessibility</h2>
 			             "a b c"
 			             "f f f";
 			       grid-template-columns: auto 1fr 20%; }
-			main > article { grid-position: b; min-width: 12em;     }
-			main > nav     { grid-position: a; /* auto min-width */ }
-			main > aside   { grid-position: c; min-width: 12em;     }
+			article { grid-position: b; min-width: 12em;     }
+			nav     { grid-position: a; /* auto min-width */ }
+			aside   { grid-position: c; min-width: 12em;     }
 		</pre>
 
 		As an added bonus,