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

Documentation/dev-tools/kmemleak.rst

@@ -227,7 +227,7 @@ Testing with kmemleak-test
 --------------------------
 
 To check if you have all set up to use kmemleak, you can use the kmemleak-test
-module, a module that deliberately leaks memory. Set CONFIG_DEBUG_KMEMLEAK_TEST
+module, a module that deliberately leaks memory. Set CONFIG_SAMPLE_KMEMLEAK
 as module (it can't be used as built-in) and boot the kernel with kmemleak
 enabled. Load the module and perform a scan with::