configs/pc: fix grub root device in help

The rootfs partition is the first partition, not the second one: the
Grub configuration file already contains root=/dev/sda1 by default,
and it should be changed to root=/dev/vda1 is Qemu emulation is used.

Signed-off-by: Cherniaev Andrei <dungeonlords789@naver.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/pc/readme.txt

@@ -42,8 +42,8 @@ Emulation in qemu (BIOS)
 1. Edit grub-bios.cfg
 
   Since the driver will show up in the virtual machine as /dev/vda,
-  change board/pc/grub-bios.cfg to use root=/dev/vda2 instead of
-  root=/dev/sda2. Then rebuild grub2 and the image.
+  change board/pc/grub-bios.cfg to use root=/dev/vda1 instead of
+  root=/dev/sda1. Then rebuild grub2 and the image.
 
 2. Run the emulation with: