Back/Forward navigation broken in navigation-in-elements.md sample code

[sentience]

The How do I manage URL from a Browser.element? guide contains a bug in the sample code that will prevent browser Back/Forward buttons from working.

The popstate event listener is incorrectly attached to the document:

// Inform app of browser navigation (the BACK and FORWARD buttons)
document.addEventListener('popstate', function () {
    app.ports.onUrlChange.send(location.href);
});

This should be added to the window to work:

// Inform app of browser navigation (the BACK and FORWARD buttons)
window.addEventListener('popstate', function () {
    app.ports.onUrlChange.send(location.href);
});