w1: fix w1_ds2438 documentation

The previous documentation was wrongly stating about the order
of magnitude of CONVERT_V result files contents (vad, vdd).
This commit is correcting this.

Reported-by: Adam Stolarczyk <adam@stolarczyk.net.pl>
Signed-off-by: Mariusz Bialonczyk <manio@skyboo.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>