Update reverse-proxy.md 反向代理http3兼容

[lutinglt]

nginx 添加 http3 请求头, add_header Alt-Svc 'h3=":$server_port"; ma=86400'; 反向代理使用 $http_host 会导致 alist 报错 400 Bad Request: missing required Host header 使用 $host:$server_port 替代, $server_port 是 nginx 监听的端口

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