atmel: fix xplained documentation

Documentation is referring to nandflash.txt which doesn't exist.
Instead, readme.txt provides the instructions to flash the NAND storage.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/atmel/xplained/readme.txt

@@ -11,7 +11,7 @@ Note that the default Buildroot configuration is prepared to boot from
 an SD card: the first stage bootloader, second stage bootloader,
 kernel image and root filesystem are all located on the SD card. Some
 adjustments in the bootloader configuration will be needed to boot
-from NAND flash, see board/atmel/nandflash.txt
+from NAND flash, see board/atmel/readme.txt
 
 Configuring and building Buildroot
 ----------------------------------