boards/raspberrypi: fix rootfs.tar path in readme

The rootfs.tar file is located under ./output/images/.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/raspberrypi/readme.txt

@@ -132,7 +132,7 @@ rootfs, there are further steps to do.
 Extract (as root!) the contents of the rootfs.tar archive into the second
 partition you created above:
 
-  $ sudo tar xf rootfs.tar -C /mnt/mountpointroot
+  $ sudo tar xf ./output/images/rootfs.tar -C /mnt/mountpointroot
 
 Finish
 ======