docs: Fix adjtime documentation

docs: Fix adjtime documentation

The first line of the adjtime file is made of three numbers (see=20
hwclock.c):
	- a drift factor as a decimal float
	- the time of last adjust as a decimal integer
	- a zero (for compatibility) as a decimal float.

but both man pages (hwclock.8 and adj_time.5) tell that the third
number is a decimal integer.

Of course this is harmless if somebody edits the adjtime file with
"0"=20 as the third number: it will be correctly read by hwclock
anyway.  But if for some reason, a program reads the adjtime file and
expects an integer, it will fail, because hwclock writes O.OOOO0O as
the third=20 number.

Signed-off-by:: Pierre Labastie <pierre.labastie@neuf.fr>
Signed-off-by: Karel Zak <kzak@redhat.com>