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/hierarchical-dependency-injection/e2e/src/app.e2e-spec.ts

@@ -7,7 +7,7 @@ describe('Hierarchical dependency injection', () => {
   });
 
   describe('Heroes Scenario', () => {
-    let page = {
+    const page = {
       heroName: '',
       income: '',