open.2: Fix bug in linkat(2) call example

open.2: Fix bug in linkat(2) call example

AT_EMPTY_PATH works with empty strings (""), but not with NULL
(or at least it's not obvious).

The relevant kernel code is the following:

linux$ sed -n 189,198p fs/namei.c
	result->refcnt = 1;
	/* The empty path is special. */
	if (unlikely(!len)) {
		if (empty)
			*empty = 1;
		if (!(flags & LOOKUP_EMPTY)) {
			putname(result);
			return ERR_PTR(-ENOENT);
		}
	}

Reported-by: Walter Harms <wharms@bfs.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>