Doc: page_migration: fix numbering for non-LRU movable flags

The non-LRU movable flags part is not relevant to above 3 funcitons, so remove
the number 4.

Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>

Link: https://lore.kernel.org/r/20210908145215.592325-1-lixinhai.lxh@gmail.com


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