Adjust the general fill-column in doc/dir-locals.example.el

Having a general fill-column of 78 may look nice at first sight, but
if the edited text gets indented a bit afterward (such as with git
comments in 'git log'), it suddenly turns not so nice on a classic 80
columns terminal.  A fill-column of 70 will serve us better.

Reviewed-by: Rich Salz <rsalz@openssl.org>