imx6ulevk: readme: Update the rootfs type to ext4

Since commit 2f37ef48e82e3ba ("configs/imx6ulevk: Use ext4 as filesystem type")
the rootfs type is ext4, so update the readme.txt accordingly.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/freescale/imx6ulevk/readme.txt

@@ -20,7 +20,7 @@ Build all components:
 
 You will find in ./output/images/ the following files:
   - imx6ul-14x14-evk.dtb
-  - rootfs.ext2
+  - rootfs.ext4
   - rootfs.tar
   - sdcard.img
   - u-boot.imx