2D tutorial bug

[LeonVincii]

2D tutorial Chapter 12: Collision Detection has the following bug:

// If the normal is anything but Vector2.Zero, this means the bat had
// moved outside the screen edge so we should reflect it about the
// normal.
if (normal != Vector2.Zero)
{
    _batVelocity = Vector2.Reflect(_batVelocity, normal);
}

the normal here needs to be normalised because the bat can hit in the corner and results in a (1, 1) normal.

[AristurtleDev]

This was resolved by MonoGame/docs.monogame.github.io#173