Update 2d_sprite_animation.rst

[Charl-pena]

Corrected the C# code from the example in the 'Controlling the animation' section, fixing the class name used by the generic method GetNode(), as well as the string parameter.

Old:

public partial class Character : CharacterBody2D
{
    private AnimatedSprite2D _animatedSprite;

    public override void _Ready()
    {
        _animatedSprite = GetNode<AnimatedSprite>("AnimatedSprite");
    }
  ...
}

New:

public partial class Character : CharacterBody2D
{
    private AnimatedSprite2D _animatedSprite;

    public override void _Ready()
    {
        _animatedSprite = GetNode<AnimatedSprite2D>("AnimatedSprite2D");
    }
  ...
}

[skyace65]

Thanks! And congrats on your first merged PR!