xfs/447: fix typo

Fix a trivial typo that keeps barfing up:
./tests/xfs/447: line 60: /dev_null: Read-only file system

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
diff --git a/tests/xfs/447 b/tests/xfs/447
index 2f95afc..05b1b67 100755
--- a/tests/xfs/447
+++ b/tests/xfs/447
@@ -58,7 +58,7 @@
 _scratch_mount
 
 $XFS_IO_PROG -fxc "pwrite 0 4k" -c fsync \
-		-c "shutdown" $SCRATCH_MNT/foo 2>&1 > /dev_null
+		-c "shutdown" $SCRATCH_MNT/foo 2>&1 > /dev/null
 
 _scratch_unmount