Adds Context to attachTo Option Example (#1602)

Previously, the documentation for attachTo provided an example where a component was being attached to a div with an id of `root`, but without it being defined it's kind of distracting. This update explicitly defines the element and attaches it to the document for clarity.

Author: CentroDL

docs/api/options.md

@@ -301,6 +301,10 @@ When attaching to the DOM, you should call `wrapper.destroy()` at the end of you
 remove the rendered elements from the document and destroy the component instance.
 
 ```js
+const div = document.createElement('div')
+div.id = 'root'
+document.body.appendChild(div)
+
 const Component = {
   template: '<div>ABC</div>'
 }