docs: kvm: fix build warnings

Fix some small build warnings. The title underline was too short in some
cases and a code block was not indented.

Documentation/virt/kvm/api.rst:7216: WARNING: Title underline too short.

Fixes: 6dba9403 ("KVM: x86: Introduce KVM_GET_SREGS2 / KVM_SET_SREGS2")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20210722100356.635078-3-ciorneiioana@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>