refactor(docs-infra): fix docs examples for tslint rule no-angle-bracket-type-assertion (#38143)

This commit updates the docs examples to be compatible with the
`no-angle-bracket-type-assertion` tslint rule.

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/binding-syntax/src/app/app.component.ts

@@ -26,7 +26,7 @@ export class AppComponent {
 
   toggleDisabled(): any {
 
-    let testButton = <HTMLInputElement> document.getElementById('testButton');
+    let testButton = document.getElementById('testButton') as HTMLInputElement;
     testButton.disabled = !testButton.disabled;
     console.warn(testButton.disabled);
   }