docs/fingerprint: Resolve contradictory FPMCU factory flashing instructions
docs/fingerprint: Resolve contradictory FPMCU factory flashing instructions

In fingerprint-factory-requirements.md, we said we don't recommend using
"flash_fp_mcu" for mass production, but
fingerprint-factory-quick-guide.md recommended using
"update_fpmcu_firmware.py" (which uses "flash_fp_mcu").

BRANCH=none
BUG=b:234772776
TEST=view in gitiles

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: If1c15c800b00b84688bdd2f7175462ef94a269c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3687545
Reviewed-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
Reviewed-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec