docs: path-lookup: update path_to_nameidata() part

No path_to_namei() anymore, step_into() will be called.
Related commit: commit c99687a0 ("fold path_to_nameidata()
into its only remaining caller")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-3-foxhlchen@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>