Documentation/sysctl/vm.txt: Fix drop_caches bit number

Bits are usually numbered starting from zero, so 4 should be bit 2, not
bit 3.
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sysctl/vm.txt

@@ -237,7 +237,7 @@ used:
 	cat (1234): drop_caches: 3
 
 These are informational only.  They do not mean that anything is wrong
-with your system.  To disable them, echo 4 (bit 3) into drop_caches.
+with your system.  To disable them, echo 4 (bit 2) into drop_caches.
 
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