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>

Documentation/virt/uml/user_mode_linux_howto_v2.rst

@@ -679,6 +679,7 @@ Starting UML
 
 We can now run UML.
 ::
+
    # linux mem=2048M umid=TEST \
     ubd0=Filesystem.img \
     vec0:transport=tap,ifname=tap0,depth=128,gro=1 \