Update book visual interp page with fixed physics example

[RJ]

Updated the visual interp book page. key insight is this bit:

Avian's SyncPlugin does this for you, but beware if you use FixedUpdate, you need to run the SyncPlugin in PostUpdate, otherwise you won't be syncing changes correctly. Visual interp happens in PostUpdate, even if physics runs in FixedUpdate.

// Run physics in FixedUpdate, but run the SyncPlugin in PostUpdate
app
  .add_plugins(
    PhysicsPlugins::new(FixedUpdate)
      .build()
      .disable::<SyncPlugin>(),
  )
  .add_plugins(SyncPlugin::new(PostUpdate));