Fix backing class in Template Lifecycle, DOM, and Modifiers (#1751)

* Fix backing class in Template Lifecycle, DOM, and Modifiers

The previous version of this example was lacking the imports and used a
bare class instead of one which `extends Component`, and also did not
include the `@action` binding, so would simply not have worked.

* Update guides/release/components/template-lifecycle-dom-and-modifiers.md

Co-authored-by: Isaac Lee <16869656+ijlee2@users.noreply.github.com>

Author: chriskrycho

guides/release/components/template-lifecycle-dom-and-modifiers.md

@@ -266,7 +266,11 @@ The simplest way to accomplish this is by using the `did-insert` modifier from [
 ```
 
 ```js {app/components/edit-form.js}
-export default class EditForm {
+import Component from '@glimmer/component';
+import { action } from '@ember/object';
+
+export default class EditFormComponent extends Component {
+  @action
   focus(element) {
     element.focus();
   }