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