docs: add missing on:click to immutable example (#10079)

Author: fritx

documentation/examples/21-miscellaneous/02-immutable-data/MutableTodo.svelte

@@ -13,7 +13,7 @@
 
 <!-- the text will flash red whenever
 		the `todo` object changes -->
-<button bind:this={btn}>
+<button bind:this={btn} on:click>
 	{todo.done ? '👍' : ''}
 	{todo.text}
 </button>