Documentations: Fix slabinfo.c directory in vm/slub.txt

Because the place of slabinfo.c changed.So update in slub.txt.
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

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 tools/slub/slabinfo.c
+gcc -o slabinfo tools/vm/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