修复dns

[davidtall]

dns {
    upstream {
	localdns: 'udp://127.0.0.1:53'
        ispdns: 'udp://116.116.116.116:53'
        proxy_dns: 'tcp+udp://1.1.1.1:53'
    }
    routing {
        request {
        qname(geosite:cn) -> localdns
	fallback: proxy_dns
        }     
    }
}

当fallback: proxy_dns 时无法启动，报错
[2026年4月21日 GMT+8 02:56:21] daemon.err: dae[15338]: time="2026-04-21T02:56:21+08:00" level=fatal msg="failed to build DNS request routing: upstream "proxy_dns" not found; please define it in section "dns.upstream""