Fix setting a pixels slice

[Neradoc]

When setting the pixels with a slice, set the correct pixel to a value provided (not to the index).

This was discovered using adafruit_led_animation.animation.rainbow

from adafruit_led_animation.animation.rainbow import Rainbow
rainbow = Rainbow(macropad.pixels, speed=.1, period=2)
while True:
    rainbow.animate()

And tested with this (with all values of rotation)

from adafruit_macropad import MacroPad
import time

macropad = MacroPad(rotation=0)

macropad.pixels[:] = (
    (255,0,0), (128,255,0), (0,255,0), (0,255,128),
    (0,0,255), (128,0,255), (255,255,255), (0,0,0),
    (255,0,0), (128,255,0), (0,255,0), (0,255,128),
)
time.sleep(2)

for x in range(9):
    macropad.pixels.fill(0)
    macropad.pixels[x:x+4] = (
        (255,0,0), (0,255,0), (0,0,255), (255,255,255)
    )
    time.sleep(1)

[tannewt]

Thank you!