docs(aio): add class attribute to example referenced in Structural Directive guide (#19446)

See https://github.com/angular/angular/blob/master/aio/content/guide/structural-directives.md

From the structural-directives.md:
The rest of the <div>, including its class attribute, moved inside the <ng-template> element.

Maybe this made sense at one time but it has become out of sync.

PR Close #19446

aio/content/examples/structural-directives/src/app/app.component.html

@@ -6,7 +6,7 @@
 
 <blockquote>
 <!-- #docregion built-in, asterisk, ngif -->
-<div *ngIf="hero" >{{hero.name}}</div>
+<div *ngIf="hero" class="name">{{hero.name}}</div>
 <!-- #enddocregion built-in, asterisk, ngif -->
 </blockquote>
 
@@ -51,7 +51,7 @@
 <p>&lt;ng-template&gt; element</p>
 <!-- #docregion ngif-template -->
 <ng-template [ngIf]="hero">
-  <div>{{hero.name}}</div>
+  <div class="name">{{hero.name}}</div>
 </ng-template>
 <!-- #enddocregion ngif-template -->