docs: path-lookup: remove filename_mountpoint

No filename_mountpoint any more
see commit: commit 161aff1d ("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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/path-lookup.rst

@@ -652,9 +652,9 @@ restarts from the top with REF-walk.
 
 This pattern of "try RCU-walk, if that fails try REF-walk" can be
 clearly seen in functions like ``filename_lookup()``,
-``filename_parentat()``, ``filename_mountpoint()``,
-``do_filp_open()``, and ``do_file_open_root()``.  These five
-correspond roughly to the four ``path_*()`` functions we met earlier,
+``filename_parentat()``,
+``do_filp_open()``, and ``do_file_open_root()``.  These four
+correspond roughly to the three ``path_*()`` functions we met earlier,
 each of which calls ``link_path_walk()``.  The ``path_*()`` functions are
 called using different mode flags until a mode is found which works.
 They are first called with ``LOOKUP_RCU`` set to request "RCU-walk".  If