Update Phaetus hotends screw dimensions

[barthy-koeln]

As per the discussion in #77 and related update in #88, printhead screws should be M3x16, not M3x12.

There are 3 more mentions of M3x12 in Printheads/README.md:

Hotend	Compatible Parts	Mounting Hardware	Notes
Chube Compact	(Chube_Compact)	4x M3x12mm
Phaetus BMS (6 fins)	(P-BMS6)	2x M3x12mm	Phaetus has two versions of the BMS; Count the number of fins on your hotend to determine your version.
Phaetus BMS (7 fins)	(P-BMS7)	2x M3x12mm	Phaetus has two versions of the BMS; Count the number of fins on your hotend to determine your version.

I am not if these need updating as well.