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/04-10/app/shared/filter-text/filter-text.service.ts

@@ -11,9 +11,9 @@ export class FilterTextService {
     let filteredList: any[];
     if (data && props && originalList) {
       data = data.toLowerCase();
-      let filtered = originalList.filter(item => {
+      const filtered = originalList.filter(item => {
         let match = false;
-        for (let prop of props) {
+        for (const prop of props) {
           if (item[prop].toString().toLowerCase().indexOf(data) > -1) {
             match = true;
             break;

aio/content/examples/styleguide/src/05-15/app/heroes/shared/hero.service.ts

@@ -8,7 +8,7 @@ import { Hero } from './hero.model';
 @Injectable()
 export class HeroService {
   getHeroes() {
-    let heroes: Hero[] = [];
+    const heroes: Hero[] = [];
     return of(heroes);
   }
 }

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);
   }
 }

aio/content/examples/styleguide/src/07-04/app/heroes/shared/hero.service.ts

@@ -8,7 +8,7 @@ import { Hero } from './hero.model';
 @Injectable()
 export class HeroService {
   getHeroes() {
-    let heroes: Hero[] = [];
+    const heroes: Hero[] = [];
     return of(heroes);
   }
 }

aio/content/examples/styleguide/src/app/hero-data.ts

@@ -1,6 +1,6 @@
 export class HeroData {
   createDb() {
-    let heroes = [
+    const heroes = [
       { id: 1, name: 'Windstorm' },
       { id: 2, name: 'Bombasto' },
       { id: 3, name: 'Magneta' },