refactor(docs-infra): fix docs examples for tslint rule prefer-const (#38143)

This commit updates the docs examples to be compatible with the
`prefer-const` 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/forms/src/app/hero-form/hero-form.component.ts

@@ -35,7 +35,7 @@ export class HeroFormComponent {
 
   skyDog(): Hero {
     // #docregion SkyDog
-    let myHero =  new Hero(42, 'SkyDog',
+    const myHero =  new Hero(42, 'SkyDog',
                            'Fetch any object at any distance',
                            'Leslie Rollover');
     console.log('My hero is called ' + myHero.name); // "My hero is called SkyDog"