Include some description about RGBW vs RGB in ws2812 example

[jcarranz97]

Maybe it was just my lack of experience with WS2812, but when I was running the ws2812 example, it was not working as expected.

I modify the code to try to understand the logic more in deep and at the end of the day the "fix" was very simple. The WS2812 that I was using was a RGB module not an RGBW.

It would be great to add a note that this is something to take into account when trying to run the example.