[css-grid-2] Specify height in example 8 for #grid (w3c#6010)

Apply the same changes from w3c#5968 to _CSS Grid Layout Module Level 2_

Author: SiarheiBobryk

css-grid-2/Overview.bs

@@ -690,6 +690,7 @@ Grid Areas</h3>
 			                       ". a";
 			  grid-template-columns: 150px 1fr;
 			  grid-template-rows: 50px 1fr 50px;
+			  height: 100vh;
 			}
 
 			#item1 { grid-area: a }