docs: Fix formatting in formatdomain.rst

The combination of italics and the since tag does not work together.

Remove it from the paragraph about using passt with vhostuser,
as well as the parentheses around it.

Signed-off-by: Yalan Zhang <yalzhang@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>