refactor(docs-infra): fix docs examples for tslint rules related to object properties (#38143)

This commit updates the docs examples to be compatible with the
`no-string-literal`, `object-literal-key-quotes` and
`object-literal-shorthand` tslint rules.

This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.

PR Close #38143

aio/content/examples/built-in-directives/src/app/app.component.ts

@@ -46,9 +46,9 @@ export class AppComponent implements OnInit {
   setCurrentClasses() {
     // CSS classes: added/removed per current state of component properties
     this.currentClasses =  {
-      'saveable': this.canSave,
-      'modified': !this.isUnchanged,
-      'special':  this.isSpecial
+      saveable: this.canSave,
+      modified: !this.isUnchanged,
+      special:  this.isSpecial
     };
   }
   // #enddocregion setClasses
@@ -107,7 +107,7 @@ export class AppComponent implements OnInit {
   trackByItems(index: number, item: Item): number { return item.id; }
 // #enddocregion trackByItems
 
-  trackById(index: number, item: any): number { return item['id']; }
+  trackById(index: number, item: any): number { return item.id; }
 
 }