slub: doc: update the slabinfo.c file path

slabinfo.c has been moved from Documentaion/vm/ to
tools/slub/ by commit:0d24db33

Update the slub.txt doc to reflect this change too.

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>