MAINTAINERS: Update path to the livepatch documentation

MAINTAINERS: Update path to the livepatch documentation

Commit d661611d08 "docs/markdown: Switch to using pandoc, and fix
underscore escaping" converted the livepatch documentation from markdown
to pandoc.

Update MAINTAINERS to reflect the change so the correct maintainers are
CCed to the patches.

Fixes: d661611d08 ("docs/markdown: Switch to using pandoc, and fix underscore escaping")
Signed-off-by: Julien Grall <julien@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>