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/service-worker-getting-started/e2e/src/app.e2e-spec.ts

@@ -15,7 +15,7 @@ describe('sw-example App', () => {
   });
 
   it('should display the Angular logo', () => {
-    let logo = element(by.css('img'));
+    const logo = element(by.css('img'));
     page.navigateTo();
     expect(logo.isPresent()).toBe(true);
   });