Change flash instructions to devkit image instead of devkit-test

Thought the script used for flashing should be flash_librem5-devkit.lst after I found the following discussion in the devkit matrix channel:

joshfleming 04:42 Mmdz0: In reply to your question about test and non-test Guido pointed to me out that the test image is just for testing and is used to verify hardware works. There's no GUI that I can see in the test image. The other image has a GUI and is the main image to work from.

mdz0 thanks joshfleming. I was confused because docs say to use uuu_scripts/flash_librem5-devkiti-test.lst which installs devkit-test...

joshfleming That confused me at first too. I suspect that documentation was originally written for people doing the hardware verification testing before the dev kits were sent out.