docs: Fix note about the required Postfix rights

docs: Fix note about the required Postfix rights

The permissions for the user running the postfix process are
not the ones used for external file or command delivery by default.
The ones defined by default_privs are (in case the aliases(5) file
that is owned by root was being used). A privileged user or the
postfix owner should not be used in this case.

See http://www.postfix.org/postconf.5.html#default_privs and
local(8).

Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>