docs: kmemleak: adjust to config renaming

docs: kmemleak: adjust to config renaming

Commit c87db8ca0902 ("kmemleak-test: fix kmemleak_test.c build logic")
essentially renames the config DEBUG_KMEMLEAK_TEST to SAMPLE_KMEMLEAK, but
misses to adjust the documentation.

Adjust kmemleak documentation to this config renaming.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230414061241.12754-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>