docs/maintain: more minor inaccuracies (fixed)

again, these were left over from a previous audit.
lbmk now uses a 1-argument design for commands,
rather than 1-argument; e.g. ./build boot roms
is now ./build roms.

there were a few stragglers left over, still referring
to the old 2-argument design. this patch fixes that.

Signed-off-by: Leah Rowe <leah@libreboot.org>