Add the missing PHP opening tag to the Volt class-based example

[sulimanbenhalim]

The class-based example closes with ?> but never opens with <?php, so copying it verbatim gives you a file that prints the PHP as text.

Four snippets in .ai/ contain ?>. The other three all open properly:

livewire/4  Single-File Component   <?php ... ?>
folio       Render Hook Example     <?php ... ?>
volt        Functional Component    <?php ... ?>
volt        Class-based Component          ... ?>

The tagless snippets elsewhere are fragments and have no ?> either, so they're consistent. This one is the only unbalanced snippet in the repo.