Allow readwrite as an alias for rw in setting sequential read/write

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/HOWTO b/HOWTO
index e4614ea..5622349 100644
--- a/HOWTO
+++ b/HOWTO
@@ -310,7 +310,7 @@
 			write		Sequential writes
 			randwrite	Random writes
 			randread	Random reads
-			rw		Sequential mixed reads and writes
+			rw,readwrite	Sequential mixed reads and writes
 			randrw		Random mixed reads and writes
 
 		For the mixed io types, the default is to split them 50/50.