我想访问本地网站的指定uri时，实际访问的是另一网站，于是使用了nginx作为代理

nginx相关配置如下
server {

listen 80;
server_name www.example.com;
.....

location /foo/ {
    proxy_store off;
    proxy_redirect off;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Referer 'no-referrer-when-downgrade';
    proxy_set_header User-Agent 'Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebkit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.62 Safari/537.36';
    proxy_pass https://api.example.com/foo/;
}

}

nginx 启动时报错 host not found in upstream 'api.example.com'