Update PC-flash.md (#1555)
Update PC-flash.md (#1555)

* Update PC-flash.md

Added two notes, to help with issues while flashing the rig, the second of which is from @Dave9111 on ITB.
https://gitter.im/nightscout/intend-to-bolus?at=5e1dc959cb2aaa2d7838cf5d

* Clarify to hold down power button for 10s

* changed couple of seconds to 10

* clarified it's the power button

Co-authored-by: Scott Leibrand <69fdce2fe72fc0d1a2901c45776b5ae52ab0877d@gmail.com>
openaps/docs