Fix bug on tutorial step 14d.

Line 19 was accessing a "hovering" variable that hadn't been initialized in the top script tag. This just initializes the variable.

site/content/tutorial/14-composition/04-slot-props/app-a/App.svelte

@@ -1,5 +1,7 @@
 <script>
 	import Hoverable from './Hoverable.svelte';
+	
+	let hovering;
 </script>
 
 <style>