docs: path-lookup: remove filename_mountpoint

docs: path-lookup: remove filename_mountpoint

No filename_mountpoint any more
see commit: commit 161aff1d93ab ("LOOKUP_MOUNTPOINT:
fold path_mountpointat() into path_lookupat()")

Without filename_mountpoint and path_mountpoint(), the
numbers should be four & three:

"These four correspond roughly to the three path_*() functions"

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-6-foxhlchen@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>