fio: minor HOWTO fix

writev(3) should be writev(2)

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 3d6b354..0d3d775 100644
--- a/HOWTO
+++ b/HOWTO
@@ -344,7 +344,7 @@
 
 			psync 	Basic pread(2) or pwrite(2) io.
 
-			vsync	Basic readv(2) or writev(3) IO.
+			vsync	Basic readv(2) or writev(2) IO.
 
 			libaio	Linux native asynchronous io.