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/set-document-title/e2e/src/app.e2e-spec.ts

@@ -6,7 +6,7 @@ describe('Set Document Title', () => {
 
   it('should set the document title', () => {
 
-    let titles = [
+    const titles = [
       'Good morning!',
       'Good afternoon!',
       'Good evening!'