Use librem5-flash-image Repo in Dev Kit Flashing the System Image How To Guide

https://developer.puri.sm/Librem5/Development_Environment/Boards/HowTo/Flashing_eMMC.html#download-the-flashing-scripts

This section is still saying to clone from https://source.puri.sm/Librem5/librem5-devkit-tools.git when it should now be from https://source.puri.sm/Librem5/librem5-flash-image.git since this script has moved on June 30th.