rainbowcomet: add missing arg initing base class

In RainbowComet, the call to `super().__init__()` was missing the
`background_color` argument. This caused strange effects when
non-default arguments were passed.

For example, setting `reverse=True` resulted a red comet with a
tail length of 1, and setting `bounce=True` resulted in a reversed
rainbow comet.

Signed-off-by: Taylor Yu <code@argon.blue>

adafruit_led_animation/animation/rainbowcomet.py

@@ -65,7 +65,7 @@ class RainbowComet(Comet):
             self._colorwheel_step = step
         self._colorwheel_offset = colorwheel_offset
         super().__init__(
-            pixel_object, speed, 0, tail_length, reverse, bounce, name, ring
+            pixel_object, speed, 0, 0, tail_length, reverse, bounce, name, ring
         )
 
     def _set_color(self, color):