Update tutorial on component binding

Updated sample code to remove boilerplate code and removed note warning about an error that is no longer present

site/content/tutorial/06-bindings/14-component-this/app-b/App.svelte

@@ -6,4 +6,4 @@
 
 <InputField bind:this={field}/>
 
-<button on:click={() => field.focus()}>Focus field</button>
+<button on:click={field.focus}>Focus field</button>

site/content/tutorial/06-bindings/14-component-this/text.md

@@ -11,9 +11,7 @@ Just as you can bind to DOM elements, you can bind to component instances themse
 Now we can programmatically interact with this component using `field`.
 
 ```html
-<button on:click="{() => field.focus()}">
+<button on:click="{field.focus}">
     Focus field
 </button>
 ```
-
-> Note that we can't do `{field.focus}` since field is undefined when the button is first rendered and throws an error.