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>