docs: cgroup-v1: replace dead CONFIG_MEM_RES_CTRL_SWAP symbol

Commit e55b9f96 ("mm: memcontrol: drop dead CONFIG_MEMCG_SWAP
config symbol") removed MEMCG_SWAP as it was just a shorthand for
CONFIG_MEMCG && CONFIG_SWAP.

The commit also adjusted the documentation but missed reference to
CONFIG_MEM_RES_CTRL_SWAP, which was the previous name of
CONFIG_MEMCG_SWAP before renaming in commit c255a458 ("memcg: rename
config variables").

Update the reference to the current state.

Signed-off-by: Krzysztof Pawlaczyk <krzpaw@gmail.com>
Reviewed-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230728105723.251892-1-krzpaw@gmail.com