docs/misc: pvcalls: Verbatim block should be indented with 4 spaces

docs/misc: pvcalls: Verbatim block should be indented with 4 spaces

At the moment, the diagram is only indented with 2 spaces. So pandoc
will try to badly interpret it and not display it correctly.

Fix it by indenting all the block by 4 spaces (i.e an extra 2 spaces).

Fixes: d661611d08 ("docs/markdown: Switch to using pandoc, and fix underscore escaping")
Signed-off-by: Julien Grall <julien@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>