runtime(doc): update vi_diff.txt: add default value for 'flash'

runtime(doc): update vi_diff.txt: add default value for 'flash'

The "flash" option was added relatively late and seems to be exclusive
to System V. (It's not in 4.4BSD and it's not in V8 UNIX, checked [on
TUHS](https://www.tuhs.org/cgi-bin/utree.pl).) The oldest occurrence of
`flash` in "a vi" I could find is in SysV R2 for the VAX, where it
[defaults to
1](https://github.com/ryanwoodsmall/oldsysv/blob/master/sysvr2-vax/src/cmd/vi/vax/ex_data.c)
= on.

closes: #14778

Signed-off-by: Cthulhux <github@tuxproject.de>
Signed-off-by: Christian Brabandt <cb@256bit.org>