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: NRich Salz <rsalz@openssl.org>

doc/dir-locals.example.el

@@ -9,7 +9,7 @@
 
 ((nil
   (indent-tabs-mode . nil)
-  (fill-column . 78)
+  (fill-column . 70)
   )
  (c-mode
   (c-file-style . "OpenSSL-II")))