Fix bug in guidebook code (#643)

# Objective

Fix the incorrect code in the guidebook setup chapter.

# Solution

Change `visible_entity_layers.insert(layer);` to
`visible_entity_layers.0.insert(layer);`

website/book/1-getting-started/setup.md

···
 
         layer_id.0 = layer;
         visible_chunk_layer.0 = layer;
-        visible_entity_layers.insert(layer);
+        visible_entity_layers.0.insert(layer);
         pos.set([0.5, 65.0, 0.5]);
         *game_mode = GameMode::Creative;
     }