docs: path-lookup: fix unrenamed WALK_GET

docs: path-lookup: fix unrenamed WALK_GET

The symbol WALK_GET does not appears in the codebase as of
0031c06807cfa8aa. It was renamed as of 8f64fb1ccef33107. A previous
documentation update, de9414adafe4, renamed one occurrence in
path-lookup.rst, but forgot to change another occurrence later in the
file.

Fixes: de9414adafe4 ("docs: path-lookup: update WALK_GET, WALK_PUT desc")
Signed-off-by: Daniel Tang <danielzgtg.opensource@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <13011949.O9o76ZdvQC@daniel-desktop3>