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/styleguide/src/07-03/app/heroes/shared/hero.service.ts

@@ -10,7 +10,7 @@ import { Hero } from './hero.model';
 })
 export class HeroService {
   getHeroes() {
-    let heroes: Hero[] = [];
+    const heroes: Hero[] = [];
     return of(heroes);
   }
 }