Quick `composite` function typo fix

[oatmealine]

In Vips::Image.composite, modes is misspelled as mode:

def composite(images : Array(Image), modes : Array(Enums::BlendMode), **kwargs)
  options = Optional.new(**kwargs)

  Operation.call("composite", options, images.unshift(self), modes).as(Type).as_image
end

Changing mode to modes makes it work completely fine, so I've gone ahead and done so:

- Operation.call("composite", options, images.unshift(self), mode).as(Type).as_image
+ Operation.call("composite", options, images.unshift(self), modes).as(Type).as_image

[naqvis]

Thanks @oatmealine