Bugfix/sitemap.org links

Hi @kjvarga,

Just a small PR that replaces the links to `http://sitemaps.org/protocol.php` with `http://www.sitemaps.org/protocol.html`

They must have updated their website at some point, and also decided to make `www.sitemaps.org` the canonical url as anything without the `www` subdomain redirects to their main page. Sigh.