docs: virt: user_mode_linux_howto_v2.rst: fix a literal block markup

There's a missing new line for a literal block:

	.../Documentation/virt/uml/user_mode_linux_howto_v2.rst:682: WARNING: Unexpected indentation.

Fixes: 04301bf5 ("docs: replace the old User Mode Linux HowTo with a new one")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>