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/docs-style-guide/e2e/src/app.e2e-spec.ts

@@ -1,7 +1,7 @@
 import { browser, element, by } from 'protractor';
 
 describe('Docs Style Guide', () => {
-  let title = 'Authors Style Guide Sample';
+  const title = 'Authors Style Guide Sample';
 
   beforeAll(() => {
     browser.get('');