Minor fixes for the AudioController.Update() at 2D Tutorial

[nahaharo]

Fix for the MonoGame/MonoGame#8851

This code fixed two things in AudiroController.Update() at 2D game tutorial.

1. move "_activeSoundEffectInstances.RemoveAt(index)" into if block.
2. add index++ at the end of while statement.

    public void Update()
    {
        int index = 0;
        while (index < _activeSoundEffectInstances.Count)
        {
            SoundEffectInstance instance = _activeSoundEffectInstances[index];

            if (instance.State == SoundState.Stopped && !instance.IsDisposed)
            {
                instance.Dispose();
                _activeSoundEffectInstances.RemoveAt(index);
            }
            index++;
        }
    }

[ThomasFOG]

I think it's still a bit broken, see MonoGame/docs.monogame.github.io#148 (comment)

[SimonDarksideJ]

Can you review again @nahaharo / @AristurtleDev to ensure the update is the latest?

[nahaharo]

I think its okay

[AristurtleDev]

Sorry for the dealy on checking the changes.

@SimonDarksideJ The code looks good, it implements the reverse loop in each of the chapter sources.

Thanks for the contribution @nahaharo