docs: path-lookup: use bare function() rather than literals

As suggested by Matthew Wilcox and Jonathan Corbet, drop ``...``
literals around function names of this patchset.

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-14-foxhlchen@gmail.com


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