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/http/src/testing/http-client.spec.ts

@@ -90,7 +90,7 @@ describe('HttpClient testing', () => {
   });
 
   it('can test multiple requests', () => {
-    let testData: Data[] = [
+    const testData: Data[] = [
       { name: 'bob' }, { name: 'carol' },
       { name: 'ted' }, { name: 'alice' }
     ];