docs: Replace dangling references to docs/interop/qmp-intro.txt

Deletion of docs/interop/qmp-intro.txt left two dangling references
behind.  Replace them by references to docs/interop/qmp-spec.rst.

Fixes: 0ec4468f233c (docs/interop: Delete qmp-intro.txt)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240120095327.666239-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
2 files changed