Documentation: KVM: update msr.rst reference

Changeset daec8d40 ("Documentation: KVM: add separate directories for architecture-specific documentation")
renamed: Documentation/virt/kvm/msr.rst
to: Documentation/virt/kvm/x86/msr.rst.

Update its cross-reference accordingly.

Fixes: daec8d40 ("Documentation: KVM: add separate directories for architecture-specific documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5652b7f5caff3b817a660b75f1f319a2f8962380.1656234456.git.mchehab@kernel.org


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