Fixed typo in onewire generic doc

Onewire devices has 6 byte long unique serial numbers, 1 byte family
code and 1 byte CRC. Linux sysfs presents the device folder in the
form of familyID-deviceID, so CRC is not shown. The consequence is
that the device serial number is always a 12 long hex-string, but
doc says 13 in one place. This is corrected by this change.
Reference: https://en.wikipedia.org/wiki/1-Wire



Signed-off-by: Gergo Huszty <huszty.gergo@digitaltrip.hu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>