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>

Documentation/vm/00-INDEX

@@ -30,8 +30,6 @@ page_migration
 	- description of page migration in NUMA systems.
 pagemap.txt
 	- pagemap, from the userspace perspective
-slabinfo.c
-	- source code for a tool to get reports about slabs.
 slub.txt
 	- a short users guide for SLUB.
 unevictable-lru.txt

Documentation/vm/slub.txt

@@ -17,7 +17,7 @@ data and perform operation on the slabs. By default slabinfo only lists
 slabs that have data in them. See "slabinfo -h" for more options when
 running the command. slabinfo can be compiled with
 
-gcc -o slabinfo Documentation/vm/slabinfo.c
+gcc -o slabinfo tools/slub/slabinfo.c
 
 Some of the modes of operation of slabinfo require that slub debugging
 be enabled on the command line. F.e. no tracking information will be