[docs] Fix app-a (sveltejs#145)

Learning module suppose to show shorthand vs longhand but has shorthand in both app-a and app-b.

Author: apatheticcoder

content/tutorial/03-advanced-svelte/06-classes/02-class-shorthand/app-a/src/lib/App.svelte

@@ -7,7 +7,7 @@
 	big
 </label>
 
-<div class:big>
+<div class:big={big}>
 	some {big ? 'big' : 'small'} text
 </div>