Build Hardware guide still has Pi 3B+ instead of Pi 4

[mgagvani]

Under the "Parts Needed" section the Pi 3B+ is still listed in multiple places. I think it should be replaced with the Pi 4 considering the 3B+ is not really powerful enough to inference at high rates and there is no reason to not go with a Pi 4.

Pi 4 Buying Links:
Adafruit
OKDo
Sparkfun