Update browser example to use CDN from NPM instead of GitHub

[jcfranco]

The browser example currently uses a CDN URL pointing to GitHub-hosted files. This should be replaced with a URL referencing the NPM package instead:

<!-- Latest version -->
<script
  src="https://cdn.jsdelivr.net/npm/countries-and-timezones@latest/dist/index.min.js"
  type="text/javascript"
></script>

<!-- Or specify a version -->
<script
  src="https://cdn.jsdelivr.net/npm/countries-and-timezones@3.8.0/dist/index.min.js"
  type="text/javascript"
></script>

Relying on the repository limits refactoring flexibility and requires bundling output to be checked into source control; adding unnecessary noise to pull requests. Switching to the published NPM package avoids this overhead and ensures the browser example always references production-ready code.

This would help with #88.