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/setup/e2e/src/app.e2e-spec.ts

@@ -2,7 +2,7 @@ import { browser, element, by } from 'protractor';
 
 describe('QuickStart E2E Tests', () => {
 
-  let expectedMsg = 'Hello Angular';
+  const expectedMsg = 'Hello Angular';
 
   beforeEach(() => {
     browser.get('');