Update mac-flash.md
Update mac-flash.md

1) Added reminders to connect battery before flashing to reduce risk of power glitch problems during flashing and UNIX upgrade. 
2) Updated instructions concerning screws and spacers, batteries, use of USB-C cables (no hubs, no adapters; when in doubt, try new cables)
3) Deleted a concern about using new MacBooks with USB-C ports, as issue is covered in the USB-cable information.
openaps/docs