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

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.1 b/fio.1
index c4c90b4..bcae37b 100644
--- a/fio.1
+++ b/fio.1
@@ -195,7 +195,7 @@
 .B randwrite
 Random writes.
 .TP
-.B rw
+.B rw, readwrite
 Mixed sequential reads and writes.
 .TP
 .B randrw