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

Documentation/admin-guide/cgroup-v1/memcg_test.rst

@@ -62,7 +62,7 @@ Please note that implementation details can be changed.
 
 	At cancel(), simply usage -= PAGE_SIZE.
 
-Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
+Under below explanation, we assume CONFIG_SWAP=y.
 
 4. Anonymous
 ============