space custom select sizing examples

docs/4.0/components/forms.md

@@ -1023,7 +1023,7 @@ Custom `<select>` menus need only a custom class, `.custom-select` to trigger th
 You may also choose from small and large custom selects to match our similarly sized text inputs.
 
 {% example html %}
-<select class="custom-select custom-select-lg">
+<select class="custom-select custom-select-lg mb-3">
   <option selected>Open this select menu</option>
   <option value="1">One</option>
   <option value="2">Two</option>