mm/doc: fix a literal block markup

Literal blocks with :: markup should be indented, as otherwise
Sphinx complains:

	Documentation/vm/hmm.rst:363: WARNING: Literal block expected; none found.

Fixes: f7ebd9ed ("mm/doc: add usage description for migrate_vma_*()")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>