Fixed README error for custom OEmbed modals

[thijskuilman]

The readme states that you have to use the default name filament_tiptap_grid when you want to overwrite the OEmbed Modal. This is an error: you should use the filament_tiptap_oembed default name instead:

class OEmbedAction extends Action
{
    public static function getDefaultName(): ?string
    {
        return 'filament_tiptap_oembed';
    }
    ...

I updated the readme to fix this instruction.