board/pc/readme.txt: fix output image name

The instructions listed sdcard.img as output image name but
board/pc/genimage-*.cfg generate it named as disk.img instead.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/pc/readme.txt

@@ -24,7 +24,7 @@ Bare PC sample config
 
   Write the image to a pendrive:
 
-  $ dd if=output/images/sdcard.img of=/dev/sdc; sync
+  $ dd if=output/images/disk.img of=/dev/sdc; sync
 
   Once it's done insert it into the target PC and boot.