lib/list_sort: fix kerneldoc build error

Commit 043b3f7b ("lib/list_sort: simplify and remove
MAX_LIST_LENGTH_BITS") added some useful kerneldoc info, but also broke the
docs build:

  ./lib/list_sort.c:128: WARNING: Definition list ends without a blank line; unexpected unindent.
  ./lib/list_sort.c:161: WARNING: Unexpected indentation.
  ./lib/list_sort.c:162: WARNING: Block quote ends without a blank line; unexpected unindent.

Fix the offending literal block and make the error go away.

Fixes: 043b3f7b

 ("lib/list_sort: simplify and remove MAX_LIST_LENGTH_BITS")
Cc: George Spelvin <lkml@sdf.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>