Documentation: mm: fix location of extfrag_index

/proc/extfrag_index does not exist.  This file is in debugfs.  Fix the
description of extfrag_threshold to reflect this.
Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sysctl/vm.txt

@@ -225,11 +225,11 @@ with your system.  To disable them, echo 4 (bit 3) into drop_caches.
 extfrag_threshold
 
 This parameter affects whether the kernel will compact memory or direct
-reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what
-the fragmentation index for each order is in each zone in the system. Values
-tending towards 0 imply allocations would fail due to lack of memory,
-values towards 1000 imply failures are due to fragmentation and -1 implies
-that the allocation will succeed as long as watermarks are met.
+reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in
+debugfs shows what the fragmentation index for each order is in each zone in
+the system. Values tending towards 0 imply allocations would fail due to lack
+of memory, values towards 1000 imply failures are due to fragmentation and -1
+implies that the allocation will succeed as long as watermarks are met.
 
 The kernel will not compact memory in a zone if the
 fragmentation index is <= extfrag_threshold. The default value is 500.