doc: Fix qemu unable to mount root fs for test-runner

doc: Fix qemu unable to mount root fs for test-runner

This patch fixes the kernel panic while loading the kernel image built
with the current config for test-runner.

 Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
 CPU: 0 PID: 1 Comm: swapper Not tainted 5.10.0-rc2-702835-ged93facee36e #6
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1 04/01/2014
 Call Trace:
  panic+0xee/0x27c
  mount_block_root+0x315/0x324
  ? rest_init+0x7a/0x7a
  prepare_namespace+0x128/0x157
  ? rest_init+0x7a/0x7a
  kernel_init+0x5/0xf6
  ret_from_fork+0x22/0x30
 Kernel Offset: 0x2a200000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbf)

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>