docs: kmemleak: DEBUG_KMEMLEAK_EARLY_LOG_SIZE changed names

Commit c5665868 ("mm: kmemleak: use the memory pool for early
allocations") renamed CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE to
CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE. Update the documentation reference
to reflect that.

Fixes: c5665868 ("mm: kmemleak: use the memory pool for early allocations")
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>