It's a '-' between 'qemu' and 'system', not a '_'.

Fixes: 324b2298 ("docs/system: convert Texinfo documentation to rST")
Message-Id: <20210401051829.3169438-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>