build(docs-infra): update project structure to cli@9 5/12 (`polyfills.ts`) (#36015)

Update `polyfills.ts` (`import './zone-flags.ts'` --> `import './zone-flags'`).

PR Close #36015

aio/tools/examples/shared/boilerplate/cli/src/polyfills.ts

@@ -35,7 +35,7 @@
  * will put import in the top of bundle, so user need to create a separate file
  * in this directory (for example: zone-flags.ts), and put the following flags
  * into that file, and then add the following code before importing zone.js.
- * import './zone-flags.ts';
+ * import './zone-flags';
  *
  * The flags allowed in zone-flags.ts are listed here.
  *