Update FullyFeaturedDropzoneView.svelte

I believe this should be `containerClasses` to match https://github.com/thecodejack/svelte-file-dropzone/blob/master/src/components/Dropzone.svelte#L330

Author: thejones

stories/views/FullyFeaturedDropzoneView.svelte

@@ -29,7 +29,7 @@
 <Dropzone
   on:drop={handleFilesSelect}
   accept={['image/*']}
-  containerClass="custom-dropzone">
+  containerClasses="custom-dropzone">
   <button>Choose images to upload</button>
 
   <p>or</p>